I just read https://docs.gradle.org/6.7/release-notes.html#jvm-toolchains
It looks like this will allow me to enforce the usage of Java 8 for this project (we need this version because most server hosts are still on Java 8 and there are no JRE's for later java version, so little people would be able to run the Editor otherwise).

I should test this once I am back on my Ubuntu machine (which can switch between different Java versions).

Pinning Java version won't be needed anymore in Custom Items 12 because of the CI/CD.

This is harder than I expected, so I will postpone this for later.