Enable RSpec/ImplicitSubject cop

Description of the proposal

This MR enables RSpec/ImplicitSubject cop. This cop was introduced and disabled in the upgrade to gitlab-styles 3.1.0. This is part of my process to re-enable cops that caused violations in the upgrade and can be tracked here: #42182 (closed)

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

🅰

# bad
it { is_expected.to be_truthy }

🅱

# good
it { expect(subject).to be_truthy }

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: #205933 (closed)
• 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

/label Engineering Productivity ~"Style decision" development guidelines ~"static analysis"

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