Enable builds for Ruby 4.0 and -head

[Usiel]

Similar to: wpscanteam/wpscan#1953

To make rubocop work with Ruby 4.1 (current ruby-head) we need to upgrade to a more recent version, which in turn requires a few changes to fix rubocop warnings.

/home/runner/work/wpscan/wpscan/vendor/bundle/ruby/4.1.0+1/gems/rubocop-1.26.1/lib/rubocop/cop/style/parallel_assignment.rb:3: warning: tsort used to be loaded from the standard library, but is not part of the default gems since Ruby 4.1.0.
You can add tsort to your Gemfile or gemspec to fix this error.
bundler: failed to load command: rubocop (/home/runner/work/wpscan/wpscan/vendor/bundle/ruby/4.1.0+1/bin/rubocop)

[coveralls]

coverage: 99.851% (-0.07%) from 99.925%
when pulling 53d9f7d on Usiel:usielriedl/add-ruby-4-build
into e2fd720 on wpscanteam:master.

[coveralls]

coverage: 99.851% (-0.07%) from 99.925%
when pulling 53d9f7d on Usiel:usielriedl/add-ruby-4-build
into e2fd720 on wpscanteam:master.