Add rake task rubocop:check:graceful

What does this MR do and why?

This MR adds a new rake task rake rubocop:check:graceful which runs RuboCop checks in CI as usual but ignores offenses from "silenced" cop rules. We silence a cop if it is enabled has Details: grace period in the YAML configuration defined.

This new rake task is run on CI twice:

• In scheduled pipelines via update-static-analysis-cache
  • In case of silenced offenses Slack is notified
• On master or MRs with RuboCop-related changes via rubocop job
  • In case of silenced offenses Slack is NOT notified to avoid spam

See #370433 (comment 1059067801).

⚠ Silencing cop rules does not work with haml-lint. So each time we enable a cop with a grace period we need to either fix the offenses or disable the cop rule in .haml-lint.yml`.

Screenshots

update-static-analysis-cache - No offenses ✅	update-static-analysis-cache - Silenced offenses ⚠ 📣	rubocop - No offenses ✅	rubocop - Silenced offenses ⚠	rubocop - Active offense 🔴
https://gitlab.com/gitlab-org/gitlab/-/jobs/2991249901	https://gitlab.com/gitlab-org/gitlab/-/jobs/2991158530	https://gitlab.com/gitlab-org/gitlab/-/jobs/2991158530	https://gitlab.com/gitlab-org/gitlab/-/jobs/2991249938	https://gitlab.com/gitlab-org/gitlab/-/jobs/2991145665

Slack notification

How to set up and validate locally

# Run uncached to trigger potential warnings from cops - see below
# --cache false won't work with --parallel
rm -fr tmp/rubocop_cache 
bundle exec rake rubocop:check:graceful

# Enable some cop rules with TODOs and re-run

rm -fr tmp/rubocop_cache 
bundle exec rake rubocop:check:graceful

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.