Move CI workflows to setup-gradle

[steve-the-edwards]

Summary

• replace explicit gradle/actions/wrapper-validation@v3 steps with gradle/actions/setup-gradle
• pin gradle/actions/setup-gradle to the current v6 commit SHA instead of using the floating @v6 tag
• let the primary test jobs keep automatic wrapper validation through setup-gradle
• disable wrapper validation in downstream main jobs that already depend on test-ubuntu, while still using setup-gradle for Gradle setup and cache access

Testing

• parsed .github/workflows/main.yml and .github/workflows/pr-checks.yml with python + yaml.safe_load
• git diff --check

Notes

• this modernizes the workflow to the current Gradle action, avoids separate wrapper-validation steps in the publish/pages jobs, and removes a floating action tag from the workflow definitions