Add change to supported versions in release docs

Adding a reminder to update the supported versions matrix in
the metal3-docs.

docs/releasing.md

@@ -120,7 +120,13 @@ visible.
 
 [Prior art](https://github.com/metal3-io/ironic-image/pull/595)
 
-### Branches lifecycle
+### Update supported versions
+
+The supported versions matrix should be updated only later after CAPI release,
+so just update supported ironic-image release branches in
+[Version Support](https://github.com/metal3-io/metal3-docs/blob/main/docs/user-guide/src/version_support.md)
+
+## Branches lifecycle
 
 Stable branches are maintained for 6 months after their creation unless
 specified differently at the moment of the branch creation.