refactor: use librarian.yaml as single version source

[julieqiu]

Delete .librarian-version.txt and use the version field in librarian.yaml as the single source of truth for the expected librarian binary version.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.02%. Comparing base (aab1d34) to head (e691496).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4600   +/-   ##
=======================================
  Coverage   95.02%   95.02%           
=======================================
  Files         195      195           
  Lines        7458     7458           
=======================================
  Hits         7087     7087           
  Misses        371      371           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[coryan]

The pending jobs:

Are finished:

This is a bug in Cloud Build, where the notification to GitHub is lost. I will restart the builds.

[coryan]

/gcbrun