Remove the deprecated rocky8 edition (#133)

The rocky8 docker build and its default python2 version of migrid has
been deprecated. Therefore we can remove it as we will no longer
actively support this edition. Instead the rocky9 edition with its
default python3 version of migrid should be used.

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