[QA] Enable a11y checks in CI

[saw-jan]

a11y checks are disabled in #13402 due to flaky tests, see #13403

The flaky check has been fixed in #13407. but the a11y checks are still not enabled, as we are adding more checks while migrating cucumber tests to playwright which can cause the PRs to fail due to a11y violations, see #13112

TODO:

• Enable the a11y checks and fix the necessary a11y violations

[saw-jan]

CC @LukasHirt @mzner