Deploy batch compiler milestones to maven central

Now that **batch-compiler is a separate jar**/project it would be great to at least deploy MILESTONES **of the single jar** to maven central, e.g. we can't ship Java 21 support unless the december release with Tycho what is a bit annoying.

So is there really any compelling reason to not do it (beside of we never did it before)?

WDYT @merks @iloveeclipse @akurtakov

Start by locating the batch-compiler project and its existing Tycho or artifact publication configuration. Check how the single batch-compiler jar is built and released, then determine the Maven Central requirements for milestone versions. Done means a milestone jar can be published to Maven Central without waiting for the December release.

Written by the indexing model from the issue text.