chore: remove unused modules

The Libraries BOM has moved to the java-cloud-bom repository.

The dashboard from the dashboard module is no longer used. Let's stop
the periodic build job.

This change also fixes the CI of the Gradle plugin.
The Gradle plugin is published to Gradle plugin portal, not Maven
Central. https://plugins.gradle.org/plugin/com.google.cloud.tools.linkagechecker

Author: suztomo

README.md

@@ -3,14 +3,6 @@
 This project explores common infrastructure and best practices for open source
 Java projects for the Google Cloud Platform (GCP).
 
-# Google Cloud Platform Java Dependency Dashboard
-
-[Google Cloud Platform Java Dependency Dashboard](
-https://storage.googleapis.com/cloud-opensource-java-dashboard/com.google.cloud/libraries-bom/snapshot/index.html)
-(runs daily; work in progress) shows multiple checks on the consistency among
-Google Cloud Java libraries. For manually generating the dashboard, see
-[its README](./dashboard/README.md).
-
 # Google Best Practices for Java Libraries
 
 [Google Best Practices for Java Libraries](https://googlecloudplatform.github.io/cloud-opensource-java/)
@@ -43,6 +35,8 @@ The [GCP Libraries BOM](https://cloud.google.com/java/docs/bom) is a Bill-of-Mat
 provides consistent versions of Google Cloud Java libraries that work together
 without linkage errors.
 
+This has moved to https://github.com/googleapis/java-cloud-bom/tree/main/libraries-bom.
+
 # Development
 
 This project is built using _Maven_.
@@ -62,5 +56,3 @@ This project is built using _Maven_.
 
 This is not an officially supported Google product.
 
-
-

boms/cloud-oss-bom/pom.xml

@@ -29,7 +29,6 @@
   </licenses>
 
   <modules>
-    <module>cloud-oss-bom</module>
     <module>cloud-lts-bom</module>
     <module>upper-bounds-check</module>
     <module>integration-tests</module>