Update eslint

[clarasb]

This PR updates eslint to v9 with related dependencies and configuration and fixes some issues that got detected due to the new lint config.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.71%. Comparing base (8f121bb) to head (2e84930).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #130   +/-   ##
=======================================
  Coverage   92.71%   92.71%           
=======================================
  Files          86       86           
  Lines        1496     1496           
  Branches      225      225           
=======================================
  Hits         1387     1387           
  Misses         91       91           
  Partials       18       18           

Flag	Coverage Δ
frontend	84.86% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[forman]

Well done, many thanks!