Add release-branch CI config for 3.x

[rymsha]

Summary

Companion to the master-side bump (#213). The build action reads releaseBranch: from the workflow file on the branch being built, so this same list has to land on 3.x itself for pushes here to be classified as release builds.

Only this one workflow file is changed — no gradle.properties bump, no docgen change, no versions.json (those are master-only per the app-booster reference).

gradle.properties on this branch is intentionally version=3.2.3-SNAPSHOT (the post-v3.2.2 "Updated to next SNAPSHOT version" commit), not v3.2.2's release version, so the release tooling doesn't try to re-release the tag.

Test plan

• CI green on this PR
• After merge, the next push to 3.x is classified as a release branch by the build action

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.42%. Comparing base (fef9130) to head (f527636).

Additional details and impacted files

@@            Coverage Diff            @@
##                3.x     #214   +/-   ##
=========================================
  Coverage     91.42%   91.42%           
  Complexity      156      156           
=========================================
  Files             9        9           
  Lines           513      513           
  Branches         58       58           
=========================================
  Hits            469      469           
  Misses           23       23           
  Partials         21       21           

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