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

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1. Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
2. Ensure that the pull request title represents the desired changelog entry
3. Please describe what you did
4. Link to relevant issues in GitHub or Jira
5. Link to relevant pull requests, esp. upstream and downstream changes
6. 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!