Activate RuboCop linting more effectively

[tnir]

Goal

Use the latest version of RuboCop with the minimal changes to maintain the quality of code.

Design

• Introducing rubocop latest, but usable with Ruby 2.6 (Drop Ruby 2.6 runtime support rubocop/rubocop#11791).
• Do not touch code lib/** and spec/** (as much as possible).

Changeset

• Update rubocop gem from 1.0.0 (2020-10-21) to 1.50.2 (2023-04-17)
• Update .rubocop_todo.yml for the first time in three years (2020-10-20).

Testing

• Run rubocop lib on your local