common: update database_backend config filed docs

[Luap99]

Podman 6 removes boltdb support so the config field docs must be updated as well, or I guess just removed.
containers/podman#27387

I guess the question should we remove the field altogether so that it isn't even parsed or keep the field and then throw an error if it is set to anything other than sqlite. I think the later is better so users with an explicit database_backend = boltdb get a proper visible error and don't wonder why things won't work all of the sudden.

[mheon]

IMO, keep it but drop the documentation. I can see us needing to add another DB for a wierd use-case eventually (probably still SQL, but maybe a dedicated server like Postgres instead of SQLite?).

[Luap99]

but maybe a dedicated server like Postgres instead of SQLite?).

Don't give people ideas!