Remove references to master branch and use main instead

[strangelookingnerd]

I noticed that there are still some references to the "master" branch that has been removed.
For the workflows this is not too important but the badges in the README were broken because of this.

Testing done

Checked if README renders badges correctly.

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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

Merging #227 (3a18e96) into main (3893571) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main     #227   +/-   ##
=========================================
  Coverage     50.00%   50.00%           
  Complexity       17       17           
=========================================
  Files             3        3           
  Lines            76       76           
  Branches          5        5           
=========================================
  Hits             38       38           
  Misses           31       31           
  Partials          7        7           

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[uhafner]

Thanks for fixing!

(I already have those changes locally. I am bulk changing those things using a script and global replace. And now I am stepping into each of my plugins to make the same transitions step by step...)