Build on latest Java, test through lowest possible

[TWiStErRob]

• Based on / closes Upgrade mockito now that the minimum Java is 11 on CI #247.
• Gradle: always 19 (20 not possible yet because support is added in 8.3)
• JDK compiler: 19 (inherited from Gradle)
• Production and test JVM bytecode: always 8
• Unit tests: always compiled and executed on 11
• Compat tests: whatever Stutter sets as per matrix
• E2E tests: executed on matrix of 3 Java versions for runtime

See https://jakewharton.com/build-on-latest-java-test-through-lowest-java/

Testing: actions executed on this commit show the Java version as failed tests.

[szpak]

I haven't got through all the changes, but a quick comment here. E2E tests are quite heavy and they hit Sonatype Nexus. That system "occasionally" has performance problems and I would prefer to not give it an extra load due to our tests. I'm really glad, the Sonatype guys are ok with us testing our e2e processes as some tests, but it's rather for smoke testing (the name could be somehow misleading, in fact they are smoke tests checking the end-to-end process with the real Nexus).

Maybe we could execute the compatTests on that way (with the real binaries)? Thanks to that we would know that JDK 8 execution is still ok, but Nexus would be not affected (which is irrelevant for that test anyway, as the REST calls on the server side shouldn't be JDK specified).

However, I'm not sure, if it would be easy to work with the compatTest to make it possible to execute them in that mode on CI (but still by default with the current JDK 11+, as we rather don't want to force developers to install all JDK 8, 11 and 17 on their machines*). WDYT?

* - I see nexusPublishPlugin.test.java which might be helpful for the default (non-CI) mode.

[TWiStErRob]

To reduce the load, we can reduce the matrix for push, but I think at least right before release it should be executed on all versions via workflow_dispatch. (Conditional CI matrix is possible without too much hassle.)

compatTests are by definition separated by Java versions, the task name contains it.
foojay-resolver does the installing on dev machines as well as CI, so it's not a big deal to have any version. That said I added auto for devs just to make it more native to the machine.