Only run rails main on newer Rubies

rails main has a requirement of Ruby >= 3.2 so all Ruby versions before
that fails with errors like

Could not find compatible versions

> Because every version of rails depends on Ruby >= 3.2.0
>   and Gemfile depends on rails >= 0,
>   Ruby >= 3.2.0 is required.
> So, because current Ruby version is = 2.7.8,
>   version solving has failed.Could not find compatible versions

.github/workflows/ruby.yml

@@ -27,6 +27,12 @@ jobs:
         exclude:
           - rails-version: '5.2.8.1'
             ruby-version: '3.3'
+          - rails-version: 'main'
+            ruby-version: '2.7'
+          - rails-version: 'main'
+            ruby-version: '3.0'
+          - rails-version: 'main'
+            ruby-version: '3.1'
 
     env:
       RAILS_VERSION: ${{ matrix.rails-version }}