Rails 7 compatibility; fix CI pipeline

[padde]

While working on #121 I encountered many failures in the CI pipeline even without making any changes. It seems that the main reason for this is that the specs are now running on Rails 7. In particular:

• ActiveSupport::Deprecation.warn raised an error
  • Calling warn on the singleton is itself deprecated¹
  • The alternative is to create our own deprecator instance, which I have implemented
• Rubocop complained about some outdated rules in .rubocop_todo.yml
  • I have removed the outdated entry
  • I also re-ran rubocop --auto-gen-config which updated some of the issue counts etc.
• A handful of specs were failing for some combinations of the CI matrix
  • Apparently in some versions the HTTP header names will be lowercase
  • I changed the matchers to match with a case-insensitive Regex instead of a capitalized string
  • This should be fine as header names are case-insensitive² anyway
• CI Pipeline was fully green after these changes, already tested on my personal fork
  • Rails 7 compatibility padde/grape-swagger-rails#2

Footnotes

1. Deprecate ActiveSupport::Deprecation singleton usage rails/rails#47354 ↩

2. "Each header field consists of a case-insensitive field name followed
   by a colon (":"), optional leading whitespace, the field value, and
   optional trailing whitespace." RFC 7230, Section 3.2 ↩