Update delete_backport_branch workflow to use github-script with proper permissions

[RileyJergerAmazon]

This PR updates the delete_backport_branch workflow to use the actions/github-script@v7 action with proper permissions instead of the SvanBoxel/delete-merged-branch@main action. This is a more modern approach that provides better security and control over the branch deletion process.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.03%. Comparing base (97defcd) to head (d31c95b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1549      +/-   ##
============================================
+ Coverage     80.93%   81.03%   +0.10%     
- Complexity     6417     6435      +18     
============================================
  Files           554      554              
  Lines         26447    26447              
  Branches       2716     2716              
============================================
+ Hits          21405    21432      +27     
+ Misses         3658     3639      -19     
+ Partials       1384     1376       -8     

Flag	Coverage Δ
plugin	81.03% <ø> (+0.10%)	⬆️

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see 11 files with indirect coverage changes

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