Remove support for older distros

Signed-off-by: Adarsh Anand <[email protected]>

config.yaml.template

@@ -4,12 +4,11 @@
 # which is recommended, you don't need to worry about the mismatch).
 #
 # Currently supported distros are:
-# - ubuntu:20.04, ubuntu:21.04, ubuntu:22.04, ubuntu:23.04
-# - debian:10, debian:11, debian:12
-# - centos:8
+# - ubuntu:22.04
+# - debian:12
 # - quay.io/centos/centos:stream9
-# - redhat/ubi8:8.8, redhat/ubi9:9.4
-# - redhat/ubi8-minimal:8.8, redhat/ubi9-minimal:9.4
+# - redhat/ubi9:9.4
+# - redhat/ubi9-minimal:9.4
 # - registry.suse.com/suse/sle15:15.4
 
 # If Distro is set to "auto", GSC detects the distro automatically by examining the supplied