Given that Derby doesn't offer any significant benefit (and most likely noone is using it…), impose significant maintenance effort and for the "quick and nice local install" it's better to use docker-compose with local database (we have example docker-compose.yaml available) we could drop support for it (it was agreed https://tigase.dev/tigase/_server/tigase-xmpp-server-docker/~issues/22#IssueComment-124851)

Plan:

1. mark Derby as deprecated in 8.4.0 (with warning when using)
2. remove Derby in 9.0.0 / 24.10