[gtkmm] CI: Switching to debian:testing

[Kjell Ahlstedt <kjellahl src gnome org>]

commit f2a9041655446ff22c03a62e516271b3cd4c5c43
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Mon Sep 13 17:17:08 2021 +0200

    CI: Switching to debian:testing
    
    See pangomm#13 and pangomm!23.

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 7de0949c..f21b235f 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,4 +1,4 @@
-image: ubuntu:devel
+image: debian:testing
 
 stages:
   - deps