refactor: use ESLint also for formatting (ESLint v9)

[susnux]

With ESLint v9 and our new configuration we can enforce the Nextcloud coding standard.
So we can replace prettier which is only somewhat close to our best practices.

Split changes into separate commits so you know why something has changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.87%. Comparing base (70d3866) to head (6929779).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1014      +/-   ##
==========================================
- Coverage   97.08%   96.87%   -0.21%     
==========================================
  Files           7        7              
  Lines         137      128       -9     
  Branches       17       17              
==========================================
- Hits          133      124       -9     
  Misses          4        4              

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