Find unused deps script and cleanup existing cruft

[billwert]

This will find any unused lines in external_dependencies.txt and version_client.txt. It won't remove them. If it finds something it will cause a build failure.

Additionally this cleans up a handful of existing old entries that I think are no longer needed. pom_file_version_scanner.ps1 still works, and grepping for the entries I removed didn't turn up anything that looked like it would break something.

Fixes #42432

[azure-sdk]

API change check

API changes are not detected in this pull request.

[billwert]

Pipeline experience in case of a failure:

/opt/hostedtoolcache/Python/3.11.10/x64/bin/python /mnt/vss/_work/1/s/eng/versioning/find_unused_dependencies.py
Unused external_dependencies.txt entries:
  springboot3_org.testcontainers:junit-jupiter-nothere

##[error]The process '/opt/hostedtoolcache/Python/3.11.10/x64/bin/python' failed with exit code 1
Finishing: Verify version text files

[billwert]

@Netyyyy @saragluna Lots of change in the Spring dependency listings here that I think is correct. I'd very much appreciate your input!

[saragluna]

@Netyyyy, will our script regenerate and append these entries here?

[Netyyyy]

No, we won't update and use these dependencies for Spring Boot2