minor-release: Document post-release update steps

After a vX.Y.0 minor release, we typically need to do a set of clean up
around (a) the upgrade guide on `main`, which now refers to the latest
stable release rather than being a placeholder for the upcoming
development release, and (b) the oldest stable branch which becomes
unmaintained following the new stable release. Document these steps.

Signed-off-by: Joe Stringer <joe@cilium.io>

Author: joestringer

.github/ISSUE_TEMPLATE/release_template_minor.md

@@ -94,6 +94,15 @@ assignees: ''
   - [ ] `echo '{"results":[{"slug":"vX.Y"}]}' > Documentation/_static/stable-version.json`.
   - [ ] Commit / amend the commit to add all of the changes above and push the PR.
   - [ ] Merge the post-release PR.
+- [ ] Update the upgrade guide and [roadmap] on the `main` branch to prepare
+      for the next development cycle. References to vX.Y should be removed, but
+      the upgrade guide structure should remain in place in order for
+      developers to document the changes necessary for the next release.
+- [ ] The oldest maintained release branch will now no longer be maintained.
+      Prepare an update PR for the `main` branch to remove references to that
+      branch. This may involve modifying documentation, code comments and
+      configuration for GitHub or Renovate.
+      - `git grep vX.Y-1`
 
 [release workflow]: https://github.com/cilium/cilium/actions/workflows/release.yaml
 [signing tags]: https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-tags
@@ -109,4 +118,5 @@ assignees: ''
 [chart workflow]: https://github.com/cilium/charts/actions/workflows/validate-cilium-chart.yaml
 [Cilium charts]: https://github.com/cilium/charts
 [default version]: https://readthedocs.org/dashboard/cilium/advanced/
+[roadmap]: https://github.com/cilium/cilium/blob/main/Documentation/community/roadmap.rst
 [security policy]: https://github.com/cilium/cilium/security/policy