java: librarian generate diff after partial release

[zhumin8]

It is expected for partial release. We only bump version for gapic-libraries-bom, but not the parent pom google-cloud-pom-parent. However, since they share the same x-version-update:google-cloud-java:current, I think there would be diff failures for the time being.

We may have to redesign the partial release process if we want to completely eliminate the diff. For example, maybe use one version for gapic-libraries-bom and another one for google-cloud-pom-parent.

Originally posted by @blakeli0 in #13447

Today, this means re-generation result won't match code base untill next snapshot PR is merged.

This generation diff happens because librarian sources google-cloud-java version (in librarian.yaml, same as versions.txt) when generating gapic-libraries-bom/pom.xml. But partial release changes leaves google-cloud-pom-parent behind.

Consider Java team now do partial releases quite often, we should fix this.
cc. @blakeli0 @JoeWang1127

[zhumin8]

During partial releases, we expect google-cloud-java version different from google-cloud-pom-parent version.
Let's:

• in versions.txt, add an entry for google-cloud-pom-parent
• update relevant tag in gapic-libraries-bom/pom.xml
• in librarian.yaml, add google-cloud-pom-parent with version. Update post generate logic to use it's version when generating gapic-libraries-bom/pom.xml

[zhumin8]

This will happen in 3 PRs:

1. in google-cloud-java, chore: add entry for google-cloud-pom-parent in versions.txt google-cloud-java#13483 introduces google-cloud-pom-parent version in versions.txt, librarian.yaml and update release-please version tags in repo, other than gapic-libraries-bom/pom.xml
2. in librarian, update logic (feat(internal/librarian/java): source google-cloud-pom-parent in pom.xml templates #6432 was reverted because step 1 is not merged)
3. in google-cloud-java, update librarian pseudo version and re-generate