ISSUE-462: Add rexml dependency for Ruby 3.0.0+

Added a ruby_version variable to rubycritic.gemspec, this variable can
be used to conditionally add dependencies.

We add rexml as a runtime dependency for environments that make use of
Ruby 3 and up

See: https://stackoverflow.com/a/65480744
Issue: #462

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 * [CHANGE] Fix test warning related to `cucumber_opts` declaration (by [@faisal][])
 * [BUGFIX] Stop using long-deprecated MiniTest module name, removed in 5.19.0 (by [@faisal][])
+* [CHANGE] Add rexml dependency for Ruby 3.0.0+ (by [@fbuys][])
 
 # v4.8.1 / 2023-05-17 [(commits)](https://github.com/whitesmith/rubycritic/compare/v4.8.0...v4.8.1)
 
@@ -432,3 +433,4 @@
 [@faisal]: https://github.com/faisal
 [@96RadhikaJadhav]: https://github.com/96RadhikaJadhav
 [@Fito]: https://github.com/Fito
+[@fbuys]: https://github.com/fbuys

rubycritic.gemspec

@@ -36,6 +36,7 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency 'parser', '>= 3.2.2.1'
   spec.add_runtime_dependency 'rainbow', '~> 3.1.1'
   spec.add_runtime_dependency 'reek', '~> 6.0', '< 7.0'
+  spec.add_runtime_dependency 'rexml', '~> 3.2', '>= 3.2.6'
   spec.add_runtime_dependency 'ruby_parser', '~> 3.20'
   spec.add_runtime_dependency 'simplecov', '>= 0.22.0'
   spec.add_runtime_dependency 'tty-which', '~> 0.5.0'