Remove the legacy rocky8 docker setup, set the default expected MIG_GIT_BRANCH to next and remove the deprecated rocky8 related ci jobs

Author: rasmunk

.github/workflows/ci-legacy.yml

@@ -39,7 +39,7 @@ jobs:
     runs-on: ubuntu-22.04
     strategy:
       matrix:
-        container_os: [rocky8, rocky9]
+        container_os: [rocky9]
         environment_profile: [development]
     steps:
       - name: Check out repository contents