CI: Replace peter-evans/create-pull-request with gh pr create

[Copilot]

Summary

Reduces CI dependency footprint by replacing peter-evans/create-pull-request action with native gh pr create command in the pin-requirements workflow.

Changes:

• Replaced action call with gh pr create using --title, --body, --base, and --head flags
• Added fallback to gh pr edit when PR already exists (preserves create-or-update behavior)
• Extracted PR title/body into shell variables to avoid duplication

Note: The delete-branch: true option is now handled by repository settings (Settings → General → Pull Requests → "Automatically delete head branches").

Release Note

Replaced peter-evans/create-pull-request GitHub Action with native gh pr create command in pin-requirements workflow.

Documentation

No documentation changes required. This is an internal CI workflow change with no user-facing impact.

Original prompt

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This section details on the original issue you should resolve

<issue_title>CI: Replace our use of peter-evans/create-pull-request with gh pr create</issue_title>
<issue_description>gh pr create should be able to do everything we currently use peter-evans/create-pull-request for, but with the nice benefit of reducing our actions dependency footprint.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes CI: Replace our use of peter-evans/create-pull-request with gh pr create #1624

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[woodruffw]

/gcbrun

[woodruffw]

/gcbrun

[woodruffw]

This may end up not working, but we can revert if it causes problems 🙂