ruby/spec no longer supports 2.7 since it is EOL

.github/workflows/spec_guards.yml

@@ -31,7 +31,7 @@ jobs:
         # Specs from ruby/spec should still run on all supported Ruby versions.
         # This also ensures the needed ruby_version_is guards are there, see spec/README.md.
         ruby:
-          - ruby-2.7
+          - ruby-3.0
           - ruby-3.1
           - ruby-3.2