Create a v3-patches branch for legacy options and have AppConfig v4 for Run3++

Follow-up from the discussion !159 (comment 5018517) in MR !159 (merged) preparing Sprucing options for the November FEST - suggestion from @cattanem:

... this could be the trigger we were looking for to FINALLY clean up AppConfig with a v4 release (see https://its.cern.ch/jira/browse/LHCBPS-1808). My suggestion would be to create a v3-patches branch which would continue the v3 legacy releases (including Python 2 support) and start with a completely clean sheet on master, where only Python 3 would be supported.