Disallow failures for Rails 6.1 on Ruby 3.0

Both are pretty stable, and the build actually passes https://github.com/rspec/rspec-rails/runs/3771356606

Author: pirj

.github/workflows/ci.yml

@@ -28,7 +28,6 @@ jobs:
 
          # Rails 6.1 builds >= 2.5
          - ruby: 3.0
-           allow_failure: true
            env:
              RAILS_VERSION: '~> 6.1.0'
          - ruby: 2.7