Streamline Github Actions Setup

[nuhkoca]

I noticed that the current workflow setup can be improved in some way.

• Target latest ubuntu version which is ubuntu-20.04
• Update actions e.g. checkout@v2, setup-java@v2, upload-artifact@v2
• Pin Java to JDK 11
• Rather than extra step to zip artifacts, we can specify multiple paths using upload-artifact action so it zips and uploads artifacts itself.
• Get rid of the step which does chmod +x .github/scripts/gradlew_recursive.sh. To let scripts have executable permissions we could use git update-index. Or we can completely remove gradlew_recursive.sh and use this approach instead.
• Apply Gradle cache so we can have faster CI/build results in next runs.