[INFRA] Rename master to main

[eseiler]

What I usually do for my fork:

• Go to the fork, settings, and rename the default branch to main
• Locally (upstream is seqan, origin is my fork):

  git branch -m master main
  git fetch upstream
  git branch -u upstream/main main
  git remote set-head upstream -a
  git fetch origin
  git branch -u origin/main main
  git remote set-head origin -a
  git fetch upstream --prune
  git fetch origin --prune

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Name	Status	Preview	Updated (UTC)
seqan3	✅ Ready (Inspect)	Visit Preview	Jan 30, 2024 2:00pm

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (dc7f41c) 98.17% compared to head (363098b) 98.17%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3230 +/- ## ======================================= Coverage 98.17% 98.17% ======================================= Files 269 269 Lines 11854 11854 ======================================= Hits 11638 11638 Misses 216 216 ```

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