Remove legacy code for old Rails and Ruby versions

[c960657]

Clean up legacy support code.

• Bump Rails version requirement to 7.0.
• Remove support code for old versions of Rails, Ruby and Mongoid.
  • Rails 7 requires Mongoid >= 7.3.4, so remove mongoid6.gemfile.
  • Hash#deep_symbolize_keys was introduced in Rails 4.
• Add test gemfiles for Rails 8.0 and 8.1.
• Enable JRuby on GitHub Action (apparently activerecord-jdbc-adapter is not yet compatiable with Rails 8, so it skips those tests).
• Enable Database Cleaner for Mongoid.
• Adjust Rakefile so tests don't run twice.

Rails 7.0 and Rails 7.1 are both EOL, but there is no additional effort in supporting those, so I kept those for now.

Ruby 3.1 is also EOL. There is just a single line in Rakefile to support this.

These requirements are very conservative and in line with the money gem. They can of course easily be bumped even more, but I suggest they stay in sync with the money gem.