chore(CI): Adjust testing matrix for Nextcloud 26 on stable26

Signed-off-by: Joas Schilling <coding@schilljs.com>

.github/workflows/integration.yml

@@ -28,7 +28,7 @@ jobs:
       matrix:
         php-versions: ['8.1']
         databases: ['sqlite', 'mysql', 'pgsql']
-        server-versions: ['master']
+        server-versions: ['stable26']
 
     name: php${{ matrix.php-versions }}-${{ matrix.databases }}-${{ matrix.server-versions }}
 
@@ -151,4 +151,4 @@ jobs:
                 repo: context.repo.repo,
                 body: comment
               })
-            }
+            }