Use Rubocop plugins, use Ruby 3.0 as minimum version everywhere

[olivier-thatch]

Just a bit of cleanup:

• use Rubocop's plugin system instead of require to silence a warning with recent Rubocop versions
  • I added version constraints to ensure the various Rubocop gems are all compatible with the new plugin system
• use 3.0 as the target Rubocop version instead of 2.7
• run Rubocop with Ruby 3.0 instead of 3.2 in CI
• use Ruby 3.0 instead of 3.1 as the oldest version in CI

This is consistent with 3.0 being the minimum required version in the gemspec.

[grape-bot]

	1 Warning
⚠️	Unless you're refactoring existing code or improving documentation, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#85](https://github.com/ruby-grape/grape-swagger-entity/pull/85): Use rubocop plugins, use ruby 3.0 as minimum version everywhere - [@olivier-thatch](https://github.com/olivier-thatch).

Generated by 🚫 Danger

[numbata]

Let's unwrap this! For example:

        include:
          # Ruby 3.1 combinations
          - ruby: '3.0'
            grape: '~> 2.0.0'
            grape_swagger: '~> 2.0.3'
            grape_entity: '~> 1.0.1'
          - ruby: '3.0'
            grape: '~> 2.0.0'
            grape_swagger: '~> 2.1.1'
            grape_entity: 'head'

          # Ruby 3.4 combinations
          - ruby: '3.4'
            grape: '~> 2.0.0'
            grape_swagger: '~> 2.0.3'
            grape_entity: '~> 1.0.1'
          - ruby: '3.4'
            grape: '~> 2.0.0'
            grape_swagger: '~> 2.0.3'
            grape_entity: 'head'
          - ruby: '3.4'
            grape: '~> 2.0.0'
            grape_swagger: 'head'
            grape_entity: '~> 1.0.1'
          - ruby: '3.4'
            grape: '~> 2.0.0'
            grape_swagger: 'head'
            grape_entity: 'head'
          - ruby: '3.4'
            grape: 'head'
            grape_swagger: 'head'
            grape_entity: '~> 1.0.1'
          - ruby: '3.4'
            grape: 'head'
            grape_swagger: 'head'
            grape_entity: 'head'

          # Ruby head combinations
          - ruby: 'head'
            grape: '~> 2.0.0'
            grape_swagger: '~> 2.0.3'
            grape_entity: '~> 1.0.1'
          - ruby: 'head'
            grape: '~> 2.0.0'
            grape_swagger: '~> 2.0.3'
            grape_entity: 'head'
          - ruby: 'head'
            grape: '~> 2.0.0'
            grape_swagger: 'head'
            grape_entity: '~> 1.0.1'
          - ruby: 'head'
            grape: '~> 2.0.0'
            grape_swagger: 'head'
            grape_entity: 'head'
          - ruby: 'head'
            grape: 'head'
            grape_swagger: 'head'
            grape_entity: '~> 1.0.1'
          - ruby: 'head'
            grape: 'head'
            grape_swagger: 'head'
            grape_entity: 'head'

This will allow us to skip, for example, (ruby=3.0, grape=head, grape-swagger=head, grape_entity=head) combination where grape-swagger-2.1.2 requires ruby version >= 3.1.