[feature] Document currently supported and/or eol'ed versions

[funkyfuture]

when working in the capacity of a system administrator i'm interested whether a (developer) requested deployment will receive security fixes (which i regard as the minimum for what i name "maintained" in the following). at the time of writing it's unclear to me whether that'd be the case for 6.0.1. right now it seems to me that three major release lines are supposed to be usable, but with the minor version part the policy/practice seems varying.

i looked around and only found a clue in SECURITY.md which seems outdated. it would be helpful to have one of these references available in a place like the Downloads webpage or the README:

• a list of currently maintained minor release lines
• a list of unmaintained minor release lines
• an elaboration of the criteria which minor release lines can be considered as maintained