docs(release): clarify roadmap & platform support

* Replace the previous roadmap with a clearer release strategy:
  - `v2.1.x` is a stability-focused maintenance branch with regular
    bi-monthly releases limited to bug fixes and security updates.
  - `v2.2.0+` is the mainline/feature branch that will introduce an
    optional `rpgp` backend for `OpenPGPv6` while preserving `GnuPG`
    as the default, fully supported backend.

* Drop support for macOS 13 from `v2.1.11` due to CI (GitHub Actions)
  discontinuation and its age; this reduces maintenance burden and
  improves security posture.

* Clarify expectations for users and contributors so stability,
  compatibility, and backend strategy are transparent.

Author: saturneric

README.md

@@ -22,11 +22,15 @@ trustworthy to your privacy life.
 
 **Planned Next Release:** 
 
-- v2.2.0+ Roadmap: Expected in the coming months, this version introduces an
-  optional rpgp backend for OpenPGPv6 (RFC 9580) support. GnuPG will remain the
-  default; however, both backends can be used simultaneously. Although rpgp will
-  initially have fewer features than GnuPG, I plan to bridge this gap through
-  gradual improvements.
+- v2.1.x (Stable): Dedicated to maximum stability and reliability, this series
+  will remain in maintenance mode with regular releases every two months. The
+  focus is strictly on bug fixes and security updates, with no new major
+  features or breaking changes planned.
+- v2.2.0+ (Mainline): Expected in the next 4–6 months, this version introduces
+  an optional rpgp backend for OpenPGPv6 (RFC 9580) support. While this branch
+  may be less stable due to its experimental nature, **GnuPG** remains our
+  primary, long-term strategic backend and will continue as the default,
+  fully-supported choice across all versions.
 
 ## Table of Contents
 

SECURITY.md

@@ -52,6 +52,10 @@ From version v2.1.8 onwards, we no longer support Ubuntu 20.04, as the LTS life
 cycle of this version had ended by then. Continuing to use this version of
 Ubuntu is therefore not safe.
 
+From version v2.1.11 onwards, we no longer support macOS 13, as it discontinued
+by GitHub Action. Given its age and the anticipated discontinuation of support
+by Apple, it is wise to drop it.
+
 ## Reporting a Vulnerability
 
 In case you wish to report a vulnerability, please avoid raising an issue