Update the CI platform guidance to include update process (#4594)

* Update the CI platform guidance to include update process

* Fix line length linter violation

Documentation/ci-platform-coverage-guidelines.md

@@ -43,3 +43,11 @@ matrix](https://github.com/dotnet/sdk/blob/main/eng/pipelines/templates/stages/v
 1. Ubuntu - Latest LTS version at the time of the release (amd64/arm64)
 1. Alpine - Latest and previous versions (amd64)
 1. AlmaLinux - Oldest version (targets lowest glibc version) (amd64)
+
+When updating the distro versions in the CI matrix as new versions are released
+and older versions reach EOL:
+
+1. Update `main` to the newer version one to two months prior to the GA/EOL date.
+    This is done to flush out any issues and to avoid destabilizing the servicing
+    branches.
+1. At the GA/EOL date, update the servicing branches.