Update instructions regarding GitHub Branch Source Plugin notifications

[ViliusS]

It looks like newest versions of GitHub Checks plugin already implements a trait to turn off duplicate notifications coming from GitHub Branch Source Plugin.

This PR just updates README instructions to use the new option instead of other plugins.

Testing done

I don't have Disable GitHub Multibranch Status Plugin or Skip Notification Trait Plugin installed. I just configured multibranch pipeline to disable status updates from GitHub Branch Source Plugin in the pipeline settings:

This produced only one commit status update in the GitHub UI:

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [ ] Link to relevant issues in GitHub or Jira
- [ ] Link to relevant pull requests, esp. upstream and downstream changes
- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c724318) 71.80% compared to head (94226c5) 71.80%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #373   +/-   ##
=========================================
  Coverage     71.80%   71.80%           
  Complexity      162      162           
=========================================
  Files            16       16           
  Lines           532      532           
  Branches         51       51           
=========================================
  Hits            382      382           
  Misses          124      124           
  Partials         26       26           

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[timja]

Thanks!