lang/python31[012]: deprecate 2026-03-31

Since the current Python upstream maintainers have not yet released
security fix releases to match 3.14.2 and 3.13.11, meaning that we have
about three unfixed security issues per 3.12/3.11/3.10 release, and the
current FreeBSD python@ team is unwilling to take approved upstream
patches individually (see PR), we need to expedite the removal of
vulnerable versions and the transition to 3.13/3.14. Deprecate all
"security support" releases of Python that are not in the bugfix phase.

PR: 291609