Drop EOL Ruby and Rails versions support

[Irving-Betterment]

Summary of changes:

• Drop end of life Ruby and Rails versions support.
• Address rubocop offenses (detailed per commit).

/task https://app.asana.com/0/1206095065335149/1206250886010422/f

[smudge]

@Irving-Betterment For this gem, I think we'd like to keep supporting older versions for the time being. It's intended as a drop-in replacement for delayed_job, and keeping more relaxed version constraints reduces the amount of work it would take to migrate over. I think we should consider dropping version support only if maintaining the older version support becomes a particular pain point.

[Irving-Betterment]

Closed as we want to keep supporting older versions for now.