[Bug] Adduser script throws errors in CI

1. Bug

What - Describe the bug

Errors in CI logs from build.py "add test users" phase.

What I expected to happen

No errors in logs

What happened instead

Traceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/rasenmaeher_api/kchelpers.py", line 222, in update_kc_user
    await self.kcadmin.a_update_user(user.kc_id, send_payload)
  File "/usr/local/lib/python3.11/site-packages/keycloak/keycloak_admin.py", line 4977, in a_update_user
    return raise_error_from_response(data_raw, KeycloakPutError, expected_codes=[204])
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/keycloak/exceptions.py", line 192, in raise_error_from_response
    raise error(
keycloak.exceptions.KeycloakPutError: 400: b'{"errors":[{"field":"modifyTimestamp","errorMessage":"updateReadOnlyAttributesRejectedMessage","params":["modifyTimestamp"]},{"field":"createTimestamp","errorMessage":"updateReadOnlyAttributesRejectedMessage","params":["createTimestamp"]}]}'

The integrations tests (test_cert_bundle) do pass.

Start with the add test users phase in build.py and the update_kc_user call at rasenmaeher_api/kchelpers.py:222. Reproduce the CI error and compare it with the passing test_cert_bundle integration test. Done means the adduser phase produces no errors in CI while the existing integration test still passes.