[libgda] CI: switch to debian stable

[Daniel Espinosa Ortiz <despinosa src gnome org>]

commit e4cc1b7678b74df61f35a11e0437c150b6cb3b07
Author: Daniel Espinosa <esodan gmail com>
Date:   Thu Sep 16 22:44:26 2021 -0500

    CI: switch to debian stable

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 45dc906f3..4f1b1db28 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -2,7 +2,7 @@
 # use the official gcc image, based on debian
 # can use verions as well, like gcc:5.2
 # see https://hub.docker.com/_/gcc/
-image: ubuntu:devel
+image: debian:stable
 
 services:
   - postgres:11