chore: use gh cli for automerge instead of action

[brian-smith-tcril]

Description

The README for the action we were using for automerge (https://github.com/peter-evans/enable-pull-request-automerge?tab=readme-ov-file#usage) states:

❗ Using this action is no longer necessary

The same functionality exists in the GitHub CLI. See the documentation here.

    - name: Enable Pull Request Automerge
      run: gh pr merge --merge --auto "1"
      env:
        GH_TOKEN: ${{ secrets.PAT }}

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Merge Checklist

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Post-merge Checklist

• Verify your changes were released to NPM at the expected version.
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[netlify]

✅ Deploy Preview for paragon-openedx-v23 ready!

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.87%. Comparing base (265240c) to head (81eefc2).

Additional details and impacted files

@@              Coverage Diff              @@
##           release-23.x    #3357   +/-   ##
=============================================
  Coverage         93.87%   93.87%           
=============================================
  Files               232      232           
  Lines              3937     3937           
  Branches            902      929   +27     
=============================================
  Hits               3696     3696           
+ Misses              237      234    -3     
- Partials              4        7    +3     

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