Support Ruby 3+ and CI with GitHub Actions

[kfprimm]

Hi,

This library has been very useful. I appreciate your efforts.

This PR:

• Adjusts the scheme registration to work with ruby 3.1+.
• Moves the CI to GH Actions, testing ruby 2.6 - 3.2.
• Fixes several minitest deprecation warnings.
• Removes deprecated has_rdoc in gemspec.
• Sets required version in gemspec to 2.6+.
• Applies a little rubocop autocorrection.

Thanks.

[PikachuEXE]

Planning to release a new gem with similar name just to let people use it with newer uri versions (required by rails 8)
data-uri-ruby#1 to be merged within this month & start releasing first version

[PikachuEXE]

Released a beta
https://rubygems.org/gems/data_uri_revived/versions/0.1.1.beta2