[MNG-7870] Undeprecate G level metadata (#1224)

The modello generated classes should not be deprecated, and they are in use. 

---

https://issues.apache.org/jira/browse/MNG-7870

maven-repository-metadata/src/main/mdo/metadata.mdo

@@ -74,10 +74,7 @@ under the License.
         <field xdoc.separator="blank">
           <name>plugins</name>
           <version>1.0.0+</version>
-          <description>The set of plugins when this directory represents a "groupId" (deprecated)</description>
-          <annotations>
-            <annotation>@Deprecated</annotation>
-          </annotations>
+          <description>The set of plugins when this directory represents a "groupId".</description>
           <association>
             <type>Plugin</type>
             <multiplicity>*</multiplicity>
@@ -383,10 +380,7 @@ under the License.
     <class java.clone="deep">
       <name>Plugin</name>
       <version>1.0.0+</version>
-      <description>Mapping information for a single plugin within this group (deprecated).</description>
-      <annotations>
-        <annotation>@Deprecated</annotation>
-      </annotations>
+      <description>Mapping information for a single plugin within this group.</description>
       <comment>NOTE: plugin version is _NOT_ included here, since it is resolved using a separate algorithm in plugins' artifact.</comment>
       <fields>
         <field>

maven-repository-metadata/src/site/apt/index.apt

@@ -44,7 +44,7 @@ Maven Repository Metadata Model
        gives data about available versions (<<<latest>>>, <<<release>>>, <<<versions>>> list and <<<lastUpdated>>>),
 
  [[3]] in a "groupId/artifactId/version" snapshot artifact directory: metadata describes <<<groupId>>>, <<<artifactId>>>, <<<version>>> (base version, i.e. ending in <<<-SNAPSHOT>>>) and
-       <<<versioning>>> element that gives data about snaphot (<<<snapshot>>>, <<<lastUpdated>>> and <<<snapshotVersions>>> list). Notice that a
+       <<<versioning>>> element that gives data about snapshot (<<<snapshot>>>, <<<lastUpdated>>> and <<<snapshotVersions>>> list). Notice that a
        release artifact directory is not expected to provide metadata.
 
  []
@@ -55,10 +55,6 @@ Maven Repository Metadata Model
 
    * a {{{./repository-metadata.html}Descriptor Reference}}.
 
- Notice: data about plugins in a directory representing a groupId is deprecated and will be removed in a future Maven version.
-~~ logic behind this:
-~~ 1. MNG-7266: maven-compat will be removed from future Maven version
-~~ 2. this will remove the code that updates plugins data: see MNG-7375/MPLUGIN-384 https://maven.apache.org/ref/3.8.4/maven-compat/apidocs/org/apache/maven/artifact/repository/metadata/GroupRepositoryMetadata.html
-~~ 3. this will lead to inconsistent data: removing it will be safer/more clear
-~~ but this logic still remains to be confirmed by clear consensus of the whole team
-
+
+ For more information see this page: {{{https://maven.apache.org/repositories/metadata.html}Maven Metadata}}.
+