Backport RSpec/ExpectInLet cop to v2 branch

[bquorning]

Backport the addition of new cop RSpec/ExpectInLet added in #1885 from the main branch to the 2-x-stable branch.

We want to get as much feedback as possible on new cops before (eventually) enabling them when v3 is released.

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master 2-x-stable (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• Added the new cop to config/default.yml.
• The cop is configured as Enabled: pending in config/default.yml.
• The cop is configured as Enabled: true in .rubocop.yml.
• The cop documents examples of good and bad code.
• The tests assert both that bad code is reported and that good code is not reported.
• Set VersionAdded: "<<next>>" in default/config.yml.

[pirj]

Thank you!

[pirj]

Now I get it why we needed the branch.