Enable MultipleMemoizedHelpers by default

Switching the cop on by default to match RSpec recommendations from its
docs and the community RSpec Style Guide recommendations.

    `let` _can_ enhance readability when used sparingly (1,2, or
    maybe 3 declarations) in any given example group, but that can
    quickly degrade with overuse. YMMV.

https://github.com/rspec/rspec-core/blob/5d87c28bb828ba14b54a0d8cfddf044894009314/lib/rspec/core/memoized_helpers.rb#L257

https://reddit.com/r/ruby/comments/73nrhk/ama_the_authors_of_effective_testing_with_rspec_3/dnsyanp/

https://rspec.rubystyle.guide/#let-blocks

Example change to dramatically reduce the number of used memoized
helpers without resorting to methods rubocop/rubocop#8447

There is no limit to the number of memoized helpers being used in
practice:

10 lets https://github.com/forem/forem/blob/master/spec/support/api_analytics_shared_examples.rb#L2
20 lets https://github.com/chef/chef/blob/master/spec/unit/provider/systemd_unit_spec.rb#L23
30 lets https://github.com/chef/chef/blob/master/spec/unit/policy_builder/policyfile_spec.rb#L24
40 lets https://github.com/diaspora/diaspora/blob/develop/spec/integration/migration_service_spec.rb#L7
50 lets https://github.com/chef/chef/blob/master/spec/functional/resource/dsc_script_spec.rb#L71

Allowing `subject` by default, i.e. subjects don't count towards the
number of memoized helpers, since there is a dedicated
`RSpec/MultipleSubjects` cop. Combination with it provides infinite
possibilities to tweak the maximum allowed number of memoized helpers to
meet the project style.

config/default.yml

@@ -396,8 +396,8 @@ RSpec/MultipleExpectations:
 
 RSpec/MultipleMemoizedHelpers:
   Description: Checks if example groups contain too many `let` and `subject` calls.
-  Enabled: false
-  AllowSubject: false
+  Enabled: true
+  AllowSubject: true
   Max: 5
   StyleGuide: https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/MultipleMemoizedHelpers