ci: test against stable and mainline

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

Author: lmb

.github/workflows/ci.yml

@@ -178,7 +178,8 @@ jobs:
     strategy:
       matrix:
         tag:
-          - "6.8"
+          - "mainline"
+          - "stable"
           - "6.6"
           - "6.1"
           - "5.15"