Change all mentions of master to main in docs and workflows

[johannaengland]

Scope and purpose

I realized that the workflows were still referencing master instead of main.

Contributor Checklist

Every pull request should have this checklist filled out, no matter how small it is.
More information about contributing to Argus can be found in the
Development docs.

• Added a changelog fragment for towncrier
• Added/amended tests for new/changed code
• Added/changed documentation, including updates to the user manual if feature flow or UI is considerably changed
• Linted/formatted the code with ruff and djLint, easiest by using pre-commit
• The first line of the commit message continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See our how-to
• If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done
• If this results in changes in the UI: Added screenshots of the before and after
• If this results in changes to the database model: Updated the ER diagram

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Test results

    4 files    504 suites   17m 56s ⏱️
  600 tests   599 ✅ 1 💤 0 ❌
2 400 runs  2 396 ✅ 4 💤 0 ❌

Results for commit 1265dbb.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.44%. Comparing base (f8c6430) to head (1265dbb).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1657   +/-   ##
=======================================
  Coverage   79.44%   79.44%           
=======================================
  Files         129      129           
  Lines        6017     6017           
=======================================
  Hits         4780     4780           
  Misses       1237     1237           

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