Introduce rubocop-rspec to rubocop CI tests

While !5 (merged) introduced RSpec/ExampleLength and RSpec/MultipleExpectations rules to our Rubocop configuration, it did not introduce rubocop-rspec gem.

This MR gets rid of runtime warnings for these "unknown" rules by adding rubocop-rspec gem.

cf. https://gitlab.com/gitlab-org/quality/toolbox/-/jobs/962107270

$ bundle exec rubocop
Warning: unrecognized cop RSpec/ExampleLength found in .rubocop.yml
Warning: unrecognized cop RSpec/MultipleExpectations found in .rubocop.yml
Inspecting 21 files
.....................
21 files inspected, no offenses detected