Backport `RSpec/ExpectInLet` cop to v2 branch

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.

Before submitting the PR make sure the following are checked:

[x] Feature branch is up-to-date with ~master~ 2-x-stable (if not - rebase it).
[x] Squashed related commits together.
[x] Added tests.
[x] Updated documentation.
[x] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
[x] 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:

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

rubocop / rubocop-rspec