Exclude Ruby 3+ with all Rails 5.x versions

These were passing with Rails 5.0 & 5.1 gemfiles, but were already
excluded with Rails 5.2. Given Rails only officially supports Ruby 3
with Rails 6+, we don't need to test/support it with 5.x versions.

.github/workflows/ruby.yml

@@ -30,6 +30,12 @@ jobs:
           # Ruby 3.x is not supported by Rails 5.2.x
           - ruby_version: 3.0
             gemfile: gemfiles/Gemfile.rails-5.2.x
+          # Ruby 3.x is not supported by Rails 5.1.x
+          - ruby_version: 3.0
+            gemfile: gemfiles/Gemfile.rails-5.1.x
+          # Ruby 3.x is not supported by Rails 5.0.x
+          - ruby_version: 3.0
+            gemfile: gemfiles/Gemfile.rails-5.0.x
           # Ruby 2.6.x is not supported by Rails main
           - ruby_version: 2.6
             gemfile: gemfiles/Gemfile.rails-main