[gtkmm] CI: Use Ubuntu 22.04

[Kjell Ahlstedt <kjellahl src gnome org>]

commit bb4e9a94836a72103a42717b3887f932487ab69e
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed May 11 15:09:10 2022 +0200

    CI: Use Ubuntu 22.04
    
    Ubuntu 22.10 is perhaps too immature.

 .gitlab-ci.yml | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 14726433..990ea677 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,6 +1,6 @@
-# 2022-03-18: ubuntu:latest = 20.04, ubuntu:rolling = 21.10, ubuntu:devel = 22.04
+# 2022-05-11: ubuntu:latest = 22.04, ubuntu:rolling = 22.04, ubuntu:devel = 22.10
 # See https://hub.docker.com/_/ubuntu
-image: ubuntu:devel
+image: ubuntu:rolling
 
 stages:
   - deps