[glibmm] CI: Switching to debian:testing

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 1fb5a12cd955096db660aacefec6c17da21ec994
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Mon Oct 4 16:40:23 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 f2010d86..5b1ea802 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,6 +1,6 @@
 # Currently we track two versions of gcc: gcc-9 and gcc-10.
 
-image: ubuntu:devel
+image: debian:testing
 
 stages:
   - deps