Merge bitcoin#16164: doc: update release process for SECURITY.md

d7c0542 doc: update release process with SECURITY.md (Jon Atack)
e4e2b28 doc: clarify support in SECURITY.md (Jon Atack)

Pull request description:

  Follow-up to bitcoin#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.

ACKs for commit d7c054:
  practicalswift:
    ACK d7c0542
  fanquake:
    ACK bitcoin@d7c0542. This seems to make sense.

Tree-SHA512: ce0f832d9804d7bfd29f2361948d7d6a4e93004a1f57e07a95dfba056caa4d8c4552267c66e6728b689b0309f4688c2d8d59d7b0c26b838c6a30df878a69fceb

Author: laanwj

SECURITY.md

@@ -2,6 +2,8 @@
 
 ## Supported Versions
 
+Versions of Bitcoin Core that are currently supported with security updates:
+
 | Version | Supported          |
 | ------- | ------------------ |
 | 0.18    | :white_check_mark: |

doc/release-process.md

@@ -41,10 +41,12 @@ Release Process
 #### After branch-off (on master)
 
 - Update the version of `contrib/gitian-descriptors/*.yml`.
+- Update the versions in `SECURITY.md` as per the software lifecycle [maintenance policy](https://bitcoincore.org/en/lifecycle/#maintenance-period), generally bumping all up one major version.
 
 #### After branch-off (on the major release branch)
 
 - Update the versions and the link to the release notes draft in `doc/release-notes.md`.
+- Delete `SECURITY.md`.
 
 #### Before final release