ci: fix public suffix backport branch

Signed-off-by: Christoph Wurst <[email protected]>

Author: ChristophWurst

.github/workflows/update-public-suffix-list.yml

@@ -16,7 +16,7 @@ jobs:
       matrix:
         branches:
           - 'main'
-          - 'stable5.5'
+          - 'stable5.6'
 
     name: update-public-suffix-list-${{ matrix.branches }}