Move development-only gems into :development group in Gemfile and update gemspec.

[todd-a-jacobs]

Issue

• General clean-up.
• Builds on top of changes made in pull request #205.

Changelog

Added

• New unit tests that validate that the gems in the :development group aren't required for non-development use.
• New Rake task to exercise the new unit tests.

Changed

• Move development-only gems in the Gemfile to the :development group.
• Refactored the Rakefile to run the additional tests along with the existing tests.
• Updated gemspec to better support Splunk, making the REXML dependency clearer on both Ruby 2.7.5 and Ruby 3.1.0.

Description of changes

• See changelog notes above.
• Tested against both Ruby 2.7.5 and Ruby 3.1.0.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[todd-a-jacobs]

Because both merge requests are against the base branch, I can't get a clean merge of the CHANGELOG.md. Here's what I've got as the section combining both PRs:

## [unreleased]
### Added
- New unit tests that validate that the gems in the :development group
  aren't required for non-development use.
- New Rake task to exercise the new unit tests.
### Changed
- Contributor experience related to RuboCop was improved with the
  following changes:
    - New .rubocop\_todo.yml to hold RuboCop offenses and enable a clean
      run of RuboCop against the current code base.
    - A number of items in .rubocop.yml were removed to allow RuboCop to
      manage them within the .rubocop\_todo.yml file.
    - Updated RuboCop's target version to Ruby 2.7.5.
    - Updated minimum Ruby version to 2.7.5 as earlier versions are
      end-of-life.
- Move development-only gems in the Gemfile to the :development group.
- Refactored the Rakefile to run the additional tests along with the
  existing tests.
- Updated gemspec to better support Splunk, making the REXML dependency
  clearer on both Ruby 2.7.5 and Ruby 3.1.0.

[todd-a-jacobs]

@reidmorrison Would it help if I rebased to remove the actual changelog entries from this PR and from #205? Both PRs would merge cleanly if I did that.

[todd-a-jacobs]

@reidmorrison The tests are all failing because the runner is using a Bundler or RubyGems version that has issues. The message says:

Your RubyGems version (2.7.6.3) has a bug that prevents required_ruby_version from working for Bundler. Any scripts that use gem install bundler will break as soon as Bundler drops support for your Ruby version. Please upgrade RubyGems to avoid future breakage and silence this warning by running gem update --system 3.2.3

I see that you removed an explicit require of rubygems back in January, and other than trying to add a dependency on a specific version of rubygems, this may be something that needs to be fixed in the runner rather than in the code. Thoughts?