Enable RSpec/LeakyConstantDeclaration cop

Description of the proposal

This enables RSpec/LeakyConstantDeclaration cop while ignoring all current violations. This was disabled in the upgrade to gitlab-styles 3.1.0. See !21886 (merged).

From https://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/LeakyConstantDeclaration

Checks that no class, module, or constant is declared.

Constants, including classes and modules, when declared in a block scope, are defined in global namespace, and leak between examples.

If several examples may define a DummyClass, instead of being a blank slate class as it will be in the first example, subsequent examples will be reopening it and modifying its behaviour in unpredictable ways. Even worse when a class that exists in the codebase is reopened.

Anonymous classes are fine, since they don't result in global namespace name clashes.

# bad
describe SomeClass do
  OtherClass = Struct.new
  CONSTANT_HERE = 'I leak into global namespace'
end

# good
describe SomeClass do
  before do
    stub_const('OtherClass', Struct.new)
    stub_const('CONSTANT_HERE', 'I only exist during this example')
  end
end

Current Violations

15217 files inspected, 158 offenses detected

Please vote 👍 if you agree with adopting this cop, 👎 if you disagree.

Check-list

• Make sure this MR enables a static analysis check rule for new usage but ignores current offenses
• Create a follow-up issue to fix the current offenses as a separate iteration: #211580
• Mention this proposal in the relevant Slack channels (e.g. #development, #backend, #frontend)
• If there is a choice to make between two potential styles, set up an emoji vote in the MR:
  • CHOICE_A: 🅰
  • CHOICE_B: 🅱
  • Vote yourself for both choices so that people know these are the choices
• The MR doesn't have significant objections, and is getting a majority of 👍 vs 👎 (remember that we don't need to reach a consensus)
• (If applicable) One style is getting a majority of vote (compared to the other choice)
• (If applicable) Update the MR with the chosen style
• Follow the review process as usual
• Once approved and merged by a maintainer, mention it again:
  • In the relevant Slack channels (e.g. #development, #backend, #frontend)
  • (Optional depending on the impact of the change) In the Engineering Week in Review

/cc @gitlab-org/maintainers/rails-backend