Update release workflow to create PR instead of direct push

[google-labs-jules]

This change updates the GitHub Actions release workflow. Instead of directly committing and pushing updates to README.md (which bumps the version number) to the default branch, it now creates a Pull Request.

Key changes:

• Added pull-requests: write permission to the job.
• Removed manual git config, git commit, and git push commands.
• Added peter-evans/create-pull-request@v6 step to handle PR creation.
• Exported DEFAULT_BRANCH to GITHUB_ENV to be used by the PR action.

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PR created automatically by Jules for task 1614296898197960783 started by @menny

[google-labs-jules]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.90%. Comparing base (d493fc4) to head (69b2a8c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #94   +/-   ##
=======================================
  Coverage   85.90%   85.90%           
=======================================
  Files          39       39           
  Lines        1291     1291           
  Branches       87       87           
=======================================
  Hits         1109     1109           
  Misses        143      143           
  Partials       39       39           

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