Update README for current check_bundler.yml workflow

[jeremysmithco]

Part of railsbump/checker#18

This is a minor update to fix the reference to the current workflow being used to check Rails compatibility in the checker repository.

Couple notes:

• I removed the link to webhooks documentation since it seems to imply that we're using GitHub webhooks here, but the result reporter is making its own API call instead. (This used to be true, but not anymore.)
• This workflow runs bundle install, it doesn't run bundle lock. But that makes me wonder, why not? It seems like it would be more efficient if that script only called bundle lock, as it would only create the lockfile and not install the gems.

[sentry]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.88%. Comparing base (b945c45) to head (4f71b95).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #151   +/-   ##
=======================================
  Coverage   86.88%   86.88%           
=======================================
  Files          48       48           
  Lines         694      694           
=======================================
  Hits          603      603           
  Misses         91       91           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.