Drop support to Rails EOL 6.0

[stefannibrasil]

To update rspec-rails to the latest version, 6.1.x, we need to drop support to Rails EOL 6.0.x version in our appraisals list. rspec-rails 6.x requires Rails 6.1 or 7.x. See this dependabot Pull Request.

This issue is for:

• dropping Rails 6.0 from the Appraisals list
• bumping rspec-rails to 6.1.x

Those can be done in different Pull requests.

[emilford]

#337 added Rails 8 to the test matrix and dropped support for Rails 6.x.

#338 bumped rspec-rails to the latest version.