Enable RSpec/MissingExampleGroupArgument cop

Ref: #42182 (closed)

What does this MR do and why?

This MR enables the RSpec/MissingExampleGroupArgument cop, which is enabled by default in rubocop-rspec. In a follow-up MR, we can run the auto-correction which should take care of the existing offences.

Screenshots or screen recordings

# bad
describe do
end

RSpec.describe do
end

# good
describe TestedClass do
end

describe "A feature example" do
end

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.