.github: update backport to not run on backport PRs

[r1walz]

Description

Backport workflow is failing on 2.x and other versions due to missing condition on PR title.

Issues Resolved

[List any issues this PR will resolve]

Check List

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d4ff554) 63.37% compared to head (9d8c3d6) 63.37%.

Additional details and impacted files

@@           Coverage Diff           @@
##              2.x     #989   +/-   ##
=======================================
  Coverage   63.37%   63.37%           
=======================================
  Files         341      341           
  Lines       11554    11554           
  Branches     2112     2112           
=======================================
  Hits         7322     7322           
  Misses       3658     3658           
  Partials      574      574           

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