build: utilizing new java version, but still building with language level 8

[aepfli]

build runs on 2 different java versions:

• 17: all tests and code quality tools
• 8: all tests

build target is version 8, but we use java 17 to generate the packages.

We can at least update checkstyle, spotless, pmd, spotbugs to newer versions, but junit, mockito, assertj, cucumber, can not be updated as we are still running the tests with java 8. Tests are not compiled on java8 so we could update all our test dependencies too. eg. i update mockito to version 5 within this pr

Theoretically, we could not run tests with java8 and only build, to ensure api compatibility :)

as the build is strangely not working now in here - see https://github.com/open-feature-forking/java-sdk-contrib/actions/runs/13217059965 of my fork

how to test:

# java 17 and above
mvn verify

#java8 
mvn verify -Pjava8

if you want to test, that this wont build successfully on java 8 with newer features, use eg. String.isBlank() (java 11) on any string and try to build with java8 :) it will even fail with the default build, informing you, that this is not supported by the language level 8.

Additional findings:

With this approach we could build everything with Java 17, but target with the release option the desired java version. This would allow different providers to have different baselines. Eg. some providers might want to upgrade to java 11 already. Giving our providers more flexibility overall.

Todos:

• adapt release please config

[toddbaert]

So from my perspective, this:

• allows us to support newer Java versions in dev environments
• allows us to use newer Java versions in CI, in order to use later versions of various plugins
• ensures that we build Java8-compatible bytecode in our release
• ensures that we don't use any >Java8 APIs via the new maven.compiler option you added

I think this is a great path forward and a huge DX improvement. Thanks a lot!

[aepfli]

@toddbaert @chrfwow should we move forward with this, as the sdk seems to be working with the new build magic, i am pretty sure we can also move forward with this one. wdyt?

[chrfwow]

Changes look good and so does the pipeline, I think we are good to go 🚀