Fix REST datetime usage (#2420)

ibexa-types.raml: Fix DateTime and datetime

`DateTime` is a PHP class. It can't be used directly.

webapi-parser throws a "Message: 'datetime' cannot be used to name a custom type"

(cherry picked from commit fc54a1d)

docs/api/rest_api/rest_api_reference/input/ibexa-types.raml

@@ -664,10 +664,6 @@ DateRange:
     description: 'Representation of date range.'
     type: object
 
-datetime:
-    description: 'Representation of date and time.'
-    type: object
-
 Event:
     description: 'Calendar event.'
     type: BaseObject
@@ -2332,7 +2328,7 @@ ContentTypeCreate:
             description: 'If set, this value overrides the current user as creator.'
             type: any
         creationDate:
-            type: DateTime
+            type: datetime
 
 ContentTypeUpdate:
     description: 'This class is used to update a content type.'
@@ -2366,7 +2362,7 @@ ContentTypeUpdate:
             description: 'If set, this value overrides the current user as creator.'
             type: any
         modificationDate:
-            type: DateTime
+            type: datetime
 
 ViewInput:
     description: 'This class represents a View input.'
@@ -3488,10 +3484,6 @@ VatCategory:
         vatValue:
             type: number
 
-DateTime:
-    description: https://www.php.net/manual/en/class.datetime.php
-    type: object
-
 ContentUpdate:
     description: 'This class is used to update a Content.'
     type: object
@@ -3516,9 +3508,9 @@ ContentMetadataUpdate:
         ownerId:
             type: any
         publishedDate:
-            type: DateTime
+            type: datetime
         modificationDate:
-            type: DateTime
+            type: datetime
         mainLanguageCode:
             type: string
         alwaysAvailable:

docs/api/rest_api/rest_api_reference/rest_api_reference.html

@@ -131617,34 +131617,6 @@ <h6 class="heading text-gray border-bottom pb-2">Types</h6>
     Representation of date range.</span>
 
 
-    <div class="items">
-        <ul class="browser-default">
-                    </ul>
-    </div>
-                                    </div>
-                                </div>
-                            </div>
-                        </div>
-                                            <div class="modal fade" id="datetime" tabindex="-1" role="dialog" aria-labelledby="datetimeTitle" aria-hidden="true">
-                            <div class="modal-dialog" role="document">
-                                <div class="modal-content">
-                                    <div class="modal-header">
-                                        <p class="modal-title" id="datetimeTitle">datetime</p>
-                                        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
-                                            <span aria-hidden="true">&times;</span>
-                                        </button>
-                                    </div>
-                                    <div class="modal-body">
-                                        <span class="font-weight-medium">datetime</span>
-
-    <span class="red-text">*</span>
-
-(<span class="font-italic">object</span>)
- - 
-<span>
-    Representation of date and time.</span>
-
-
     <div class="items">
         <ul class="browser-default">
                     </ul>
@@ -139910,16 +139882,12 @@ <h6 class="heading text-gray border-bottom pb-2">Types</h6>
 
     <span class="red-text">*</span>
 
-(<span class="font-italic">DateTime</span>)
+(<span class="font-italic">datetime</span>)
  - 
 <span>
     </span>
 
 
-    <div class="items">
-        <ul class="browser-default">
-                    </ul>
-    </div>
                 </li>
                     </ul>
     </div>
@@ -140100,16 +140068,12 @@ <h6 class="heading text-gray border-bottom pb-2">Types</h6>
 
     <span class="red-text">*</span>
 
-(<span class="font-italic">DateTime</span>)
+(<span class="font-italic">datetime</span>)
  - 
 <span>
     </span>
 
 
-    <div class="items">
-        <ul class="browser-default">
-                    </ul>
-    </div>
                 </li>
                     </ul>
     </div>
@@ -146974,34 +146938,6 @@ <h6 class="heading text-gray border-bottom pb-2">Types</h6>
 
                 </li>
                     </ul>
-    </div>
-                                    </div>
-                                </div>
-                            </div>
-                        </div>
-                                            <div class="modal fade" id="DateTime" tabindex="-1" role="dialog" aria-labelledby="DateTimeTitle" aria-hidden="true">
-                            <div class="modal-dialog" role="document">
-                                <div class="modal-content">
-                                    <div class="modal-header">
-                                        <p class="modal-title" id="DateTimeTitle">DateTime</p>
-                                        <button type="button" class="close" data-dismiss="modal" aria-label="Close">
-                                            <span aria-hidden="true">&times;</span>
-                                        </button>
-                                    </div>
-                                    <div class="modal-body">
-                                        <span class="font-weight-medium">DateTime</span>
-
-    <span class="red-text">*</span>
-
-(<span class="font-italic">object</span>)
- - 
-<span>
-    https://www.php.net/manual/en/class.datetime.php</span>
-
-
-    <div class="items">
-        <ul class="browser-default">
-                    </ul>
     </div>
                                     </div>
                                 </div>
@@ -147146,32 +147082,24 @@ <h6 class="heading text-gray border-bottom pb-2">Types</h6>
 
     <span class="red-text">*</span>
 
-(<span class="font-italic">DateTime</span>)
+(<span class="font-italic">datetime</span>)
  - 
 <span>
     </span>
 
 
-    <div class="items">
-        <ul class="browser-default">
-                    </ul>
-    </div>
                 </li>
                             <li>
                     <span class="font-weight-medium">modificationDate</span>
 
     <span class="red-text">*</span>
 
-(<span class="font-italic">DateTime</span>)
+(<span class="font-italic">datetime</span>)
  - 
 <span>
     </span>
 
 
-    <div class="items">
-        <ul class="browser-default">
-                    </ul>
-    </div>
                 </li>
                             <li>
                     <span class="font-weight-medium">mainLanguageCode</span>