Modernize RuboCop usage, enable performance cops, add minitest and rake cops

[larouxn]

Modernizes our RuboCop stack to latest, adds cops for Minitest and Rake, and enables rubocop-performance. It's been in the Gemfile but not enabled for seemingly years. Note, this is part 1, hence the large .rubocop_todo.yml addition. Part 2 will correct much of those newly added todos via safe auto-correction.

• Safe auto-correct and manually fix RuboCop infractions and regen .rubocop.yml #2206

Note, when testing locally you'll need to update rubocop-performance if you have a previous installation.

git checkout modernize_rubocop_usage
bundle install
bundle update rubocop rubocop-performance --conservative
bundle exec rake

Requires the following (contains their commits)

• Drop support for EOL Ruby 2.7 #2204

[larouxn]

Correcting mistake made in #2207. Should use oldest support Ruby. (context)

[jneen]

@larouxn Thank you for your patience! I think there are some conflicts with the recently-merged #2202, if you wouldn't mind rebasing over that and re-running the fix. I can give it a shot as well if it's too much trouble.

[larouxn]

👋 No worries! This PR and #2206 have been rebased.

Just one quick note, it appears we have two ostruct Gemfile entries. 😅

• Remove duplicate ostruct entry from Gemfile #2212

[jneen]

Great catch, my bad! I'll take another look.

[jneen]

I'm getting this error running bundle exec rake:

Running RuboCop...
Error: Failed to load plugin `rubocop-performance` because the corresponding plugin class could not be determined for instantiation.
Try upgrading it first (e.g., `bundle update rubocop-performance`).
If `rubocop-performance` is not yet a plugin, use `require: rubocop-performance` instead of `plugins: rubocop-performance` in your configuration.

For further assistance, check with the developer regarding the following points:
https://docs.rubocop.org/rubocop/plugin_migration_guide.html

Edit: looks fixed with ~> 1.26 on rubocop-performance.