Make backend-update PR branches version-specific

[MilesCranmerBot]

This updates the backend update workflow to use a version-specific PR branch name (e.g. backend-update/v1.13.1) so new backend releases open a new PR instead of overwriting the existing one.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.25%. Comparing base (a05337c) to head (8c9307e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1155   +/-   ##
=======================================
  Coverage   94.25%   94.25%           
=======================================
  Files          21       21           
  Lines        1687     1687           
=======================================
  Hits         1590     1590           
  Misses         97       97           

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