Add rubocop to CI

[dwelch-r7]

Description

Add rubocop to github actions CI and updated the codebase to comply/or added exceptions to the rubocop config Also bumps the minimum ruby version in the gemspec to 2.5 which is the minimum ruby version currently under test in CI Recommend viewing with whitespace turned off: https://github.com/rapid7/recog/pull/579/files?diff=unified&w=1

Motivation and Context

Add RuboCop (linter) workflow Similar PR here in the recog-ruby repo: https://github.com/rapid7/recog-ruby/pull/15

How Has This Been Tested?

• Ran rubocop locally
• Ran github actions locally (via act)
• Github actions on the PR

Types of changes

Additions to CI Code linting

Checklist:

• [ ] I have updated the documentation accordingly (or changes are not required).
• [ ] I have added tests to cover my changes (or new tests are not required).
• [ ] All new and existing tests passed.