Add findBy method to Repository.stub

[sabbelasichon]

Since TYPO3 the magic find by methods are deprecated. https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/12.3/Deprecation-100071-MagicRepositoryFindByMethods.html

We are still loading our custom Repository.stub, so we should include this method in the stub as well.

[sabbelasichon]

Maybe we should not load the Repository.stub for TYPO3 12, because the core handles everything fine for us.

[sascha-egerer]

Not sure if this is still a thing - at least for TYPO3 13. Needs to be checked again.