upgrade to java 17

[markus-lmax]

which requires upgrading gradle and mockito as well

[markus-lmax]

Review comment from Daniel Milnes (@thebeanogamer):

Is there a reason you've gone with v3 of gradle/actions/setup-gradle instead of the latest v5?

I tried using the latest v5, but running that locally with https://github.com/nektos/act failed with

[Java CI with Gradle/build]   ☁  git clone 'https://github.com/gradle/actions' # ref=v5
[Java CI with Gradle/build] Unable to resolve v5: unsupported object type

Given act correctly pointed out an actual issue earlier in the process and passed after I had fixed that, I trust the tool enough to leave everything as-is. Perhaps v5 only supports later gradle versions 🤷.