CI: Linting the YAML

GitHub's editor made linting complaints about the expression of the list. This Expands the list to "bullet-point form".

The 3.0 stringifies as "3". so, in order to be consistent, quote it.

.github/workflows/ruby.yml

@@ -19,21 +19,19 @@ jobs:
       matrix:
         ruby_version: [3.1, "3.0", 2.7, 2.6, jruby]
         gemfile:
-          [
-            Gemfile,
-            gemfiles/Gemfile.rails-5.2.x,
-            gemfiles/Gemfile.rails-6.0.x,
-            gemfiles/Gemfile.rails-6.1.x,
-            gemfiles/Gemfile.rails-7.0.x,
-            gemfiles/Gemfile.rails-main,
-          ]
+          - Gemfile
+          - gemfiles/Gemfile.rails-5.2.x
+          - gemfiles/Gemfile.rails-6.0.x
+          - gemfiles/Gemfile.rails-6.1.x
+          - gemfiles/Gemfile.rails-7.0.x
+          - gemfiles/Gemfile.rails-main
         exclude:
           # Ruby 3.1 is not supported by Rails 5.2.x
           - ruby_version: 3.1
             gemfile: gemfiles/Gemfile.rails-5.2.x
 
           # Ruby 3.x is not supported by Rails 5.2.x
-          - ruby_version: 3.0
+          - ruby_version: "3.0"
             gemfile: gemfiles/Gemfile.rails-5.2.x
 
           # Ruby 2.6.x is not supported by Rails main