Mention the minimum required bob.db.base version

The bob.db.base.FileDatabase class was introduced in bob.db.base 2.2.1:

https://github.com/bioidiap/bob.db.base/commit/b3e2273d3243a7c4adaded4129c50a86927f2724

changed the description

changed milestone to %Conda-based CI

enabled an automatic merge when the pipeline for 3b9c8256 succeeds

mentioned in commit f8461b52

merged