docs: State which versions we do maintain (#4085) (#4090)

As pointed out on the community slack, we don't document what releases
are supported.

(cherry picked from commit 1f168a7)

Co-authored-by: Christian Simon <simon@swine.de>

Author: github-actions[bot]

docs/sources/release-notes/_index.md

@@ -9,6 +9,8 @@ weight: 100
 Release notes for Grafana Pyroscope are in the CHANGELOG for the release and
 listed here by version number.
 
+The last two minor versions of Grafana Pyroscope are supported and get security updates and bug fixes.
+
 Packages are linked in the [Assets section for each Pyroscope release](https://github.com/grafana/pyroscope/releases).
 
-{{< section >}}
+{{< section >}}