Mark suites for 20.3.1.0.Final release

[zakkak]

This PR was automatically generated by mandrel-release.java of graalvm/mandrel-packaging!

Please tag branch mandrel/20.3 after merging, and push the tag.

git checkout mandrel/20.3
git pull upstream mandrel/20.3
git tag -a mandrel-20.3.1.0.Final -m "mandrel-20.3.1.0.Final" -s
git push upstream mandrel-20.3.1.0.Final

where upstream is the git remote showing to https://github.com/graalvm/mandrel

Make sure to create the same tag in the mandrel-packaging repository!