Add AlmaLinux 8 support

Added AlmaLinux 8 to the supported operating systems in metadata.json.

These changes ensure that we start testing with AlmaLinux 8 and 9 instead
of CentOS 8 Stream. This update is part of our transition to better support
AlmaLinux in our Puppet modules.

See, for details: theforeman/foreman-infra#2087

metadata.json

@@ -64,6 +64,7 @@
     {
       "operatingsystem": "AlmaLinux",
       "operatingsystemrelease": [
+        "8",
         "9"
       ]
     },