chore: an entry for google-cloud-pom-parent in versions.txt

[zhumin8]

In this repo, google-cloud-pom-parent artifact version are tracked with same marker as google-cloud-java. However in partial releases, google-cloud-pom-parent are not bump together with google-cloud-java. This change is to separate and create a tracking version for google-cloud-pom-parent.
This change needs to coordinate with librarian.yaml change.
Changes in this PR:

• In versions.txt, add an entry for google-cloud-pom-parent
• In librarian.yaml, add google-cloud-pom-parent with version.
• Update pom.xml references to google-cloud-pom-parent, use the new marker
• leave out google-cloud-pom-parent in gapic-libraries-bom/pom.xml for now to avoid generation diff mid change. This file is generated from template. Will update this occurrence in same PR when updating librarian version that includes necessary changes on librarian side..

For googleapis/librarian#6411

[gemini-code-assist]

Code Review

This pull request updates the version-update tracking comments across numerous pom.xml files to reference google-cloud-pom-parent instead of google-cloud-java. Additionally, it registers the google-cloud-pom-parent module in librarian.yaml and versions.txt. I have no feedback to provide as there are no review comments.