Follow-up to #16140:
Update the release process to maintain SECURITY.md; this looks like the sort of item that can otherwise be easily overlooked during a major release
Clarify type of support in SECURITY.md
Question: If https://bitcoincore.org/en/lifecycle/#maintenance-period is still current policy, should v0.15 now be unmaintained and v0.16 EOL... seems the schedule on that page could use an update.