bump setup-gradle action to v6 in common-setup workflow

[314systems]

it resolves this warning

Warning: Node.js 20 actions are deprecated. The following actions are running on Node.js 20 and may not work as expected: gradle/actions/setup-gradle@v4. Actions will be forced to run with Node.js 24 by default starting June 2nd, 2026. Node.js 20 will be removed from the runner on September 16th, 2026. Please check if updated versions of these actions are available that support Node.js 24. To opt into Node.js 24 now, set the FORCE_JAVASCRIPT_ACTIONS_TO_NODE24=true environment variable on the runner or in your workflow file. Once Node.js 24 becomes the default, you can temporarily opt out by setting ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.83%. Comparing base (e3b385e) to head (a615994).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #608   +/-   ##
=========================================
  Coverage     97.83%   97.83%           
  Complexity      975      975           
=========================================
  Files           121      121           
  Lines          2581     2581           
  Branches        211      211           
=========================================
  Hits           2525     2525           
  Misses           19       19           
  Partials         37       37           

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