Decide and document release maintenance lifecycle

[vytas7]

See also this comment: #2409 (comment).

We ought to have a clear maintenance policy on what is considered maintained, and for how long.
Do we want to maintain anything from the "old stable", i.e., SemVer major version -1?

Some examples from other projects:

• MariaDB: MariaDB general release maintenance periods.
• SQLAlchemy: Release status.

[CaselIT]

I would also have a time bound since we usually don't release majors so often.

Say a couple of years

[vytas7]

I would also have a time bound since we usually don't release majors so often.

Say a couple of years

👍 Makes sense to highlight this trend. (For reference, we've been discussing this on Gitter too.)