Release Google.Cloud.DocumentAI.V1Beta3 version 2.0.0-beta18 (#12610)

Changes in this release:

### New features

- Support a new Layout Processor in Document AI ([commit 8912d83](https://togithub.com/googleapis/google-cloud-dotnet/commit/8912d830ea5827eac95969da3db8642505fd6253))

### Documentation improvements

- Keep the API doc up-to-date with recent changes ([commit 8912d83](https://togithub.com/googleapis/google-cloud-dotnet/commit/8912d830ea5827eac95969da3db8642505fd6253))

apis/Google.Cloud.DocumentAI.V1Beta3/Google.Cloud.DocumentAI.V1Beta3/Google.Cloud.DocumentAI.V1Beta3.csproj

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <Version>2.0.0-beta17</Version>
+    <Version>2.0.0-beta18</Version>
     <TargetFrameworks>netstandard2.0;net462</TargetFrameworks>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <Description>Recommended Google client library to access the Google Document AI API (v1beta3), which is a service to parse structured information from unstructured or semi-structured documents using state-of-the-art Google AI such as natural language, computer vision, translation, and AutoML.</Description>

apis/Google.Cloud.DocumentAI.V1Beta3/docs/history.md

@@ -1,5 +1,15 @@
 # Version history
 
+## Version 2.0.0-beta18, released 2024-04-19
+
+### New features
+
+- Support a new Layout Processor in Document AI ([commit 8912d83](https://github.com/googleapis/google-cloud-dotnet/commit/8912d830ea5827eac95969da3db8642505fd6253))
+
+### Documentation improvements
+
+- Keep the API doc up-to-date with recent changes ([commit 8912d83](https://github.com/googleapis/google-cloud-dotnet/commit/8912d830ea5827eac95969da3db8642505fd6253))
+
 ## Version 2.0.0-beta17, released 2024-03-26
 
 ### New features

apis/apis.json

@@ -2142,7 +2142,7 @@
     },
     {
       "id": "Google.Cloud.DocumentAI.V1Beta3",
-      "version": "2.0.0-beta17",
+      "version": "2.0.0-beta18",
       "type": "grpc",
       "productName": "Cloud Document AI",
       "productUrl": "https://cloud.google.com/solutions/document-ai",