[sdk-platform-java] Improve the process of discovering source code for a specific gax version

**Is your feature request related to a problem? Please describe.**
When debugging customer issues, a service team usually starts out with a client can gax version as they were sent by the api_client header. Ever since the mono repo merge it extremely toilsome to find the source code the a specific version of gax. I generally do a manual bisect of the sdk platform tags and check the pom.xml file of gax until I find the correct version.

**Describe the solution you'd like**
Align the version of sdk-platform tags and gax version.

**Describe alternatives you've considered**
Alternatively maintain a mapping in documentation.