java: rethink how libraries bom version is configured

[zhumin8]

Now that libraries-bom is migrated inside of google-cloud-java(googleapis/google-cloud-java#13498), perhaps move its version from default into libraries list, so that release-please can track and update its version when releasing.

https://github.com/googleapis/google-cloud-java/blob/b56f9b854f9ba110606b2f123b989d6ca758c6a2/librarian.yaml#L50-L52

default:
  java:
    libraries_bom_version: 26.83.0

[zhumin8]

Counter argument:
release-please updates it's version with release PR, it creates a generation diff situation. (release-please PR does not run re-generate)

release-please bump versions, maybe let release-please update README instead of/in addition to librarian generate.
It's really the version of the library and version of libraries BOM specified in README.md. e.g, https://screenshot.googleplex.com/B8ViixyThAdysNb

Effort: 1-2 days (including coordinate with README template change and push to google-cloud-java)

[JoeWang1127]

release-please bump versions, maybe let release-please update README instead of/in addition to librarian generate.

I think this is a valid approach.

[zhumin8]

A cleaner and easier change is to coordinate this with the ongoing effort to move away from owlbot.py postprocessing.

To clarify the path forward and blockers/dependencies:

• move to librarian-based README generation, instead of owlbot.py (part of on-going https://github.com/googleapis/librarian/milestone/142).
• let release-please update README (tracked in java: readme is not updated with release PR #6684)
• remove config libraries_bom_version and logic, this value is currently passed down via env var to owlbot.py postprocessing (tracked in this ticket)