Linkage Monitor fails to detect linkage errors in gax-java

[suztomo]

Linkage Monitor fails to detect linkage errors in gax-java.

Linkage Monitor has capability to replace a BOM in the libraries BOM. However, we no longer specify gax-bom in it. Therefore current it does not detect anything in gax-java's repository.

This bug is only applicable for gax-java repository, which relies on gax-java BOM to tell locally installed artifacts. Other Maven projects in Yoshi repositories, which produces pom.xml files in project directories, are not affected.

[suztomo]

1. Read pom.xml (done) and build.gradle. Does this exist?
2. Check target directories. gax-java does not generate pom files.
3. Each repository to have a list of artifacts.
   not in ci.yaml : https://github.com/googleapis/java-spanner/blob/master/.github/workflows/ci.yaml for synthtool
   independent files in each repository. 80+ repositories... Respect the current working procedure so that we only need files for special repositories.

I'll do with a minor release.