Force use of symetric JWT algorithm for internal logs server

[ashb]

Since the logs server was hard-coded to use a secret_key, if a user specified
the api_auth.jwt_algorithm config section it would have interferred and
caused this error:

Traceback (most recent call last):
  File "/usr/local/lib/python3.12/site-packages/airflow/utils/log/file_task_handler.py", line 567, in _read_from_logs_server
    response = _fetch_logs_from_service(url, rel_path)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/airflow/utils/log/file_task_handler.py", line 112, in _fetch_logs_from_service
    headers={"Authorization": generator.generate({"filename": log_relative_path})},
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/airflow/api_fastapi/auth/tokens.py", line 458, in generate
    return jwt.encode(claims, self.signing_arg, algorithm=self.algorithm, headers=headers)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/jwt/api_jwt.py", line 78, in encode
    return api_jws.encode(
           ^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/jwt/api_jws.py", line 170, in encode
    key = alg_obj.prepare_key(key)
          ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/jwt/algorithms.py", line 738, in prepare_key
    raise InvalidKeyError(
jwt.exceptions.InvalidKeyError: Expecting a EllipticCurvePrivateKey/EllipticCurvePublicKey. Wrong key provided for EdDSA algorithms

The fix is to force a use of HS512 (the same one you would get if you didn't
specify an algorithm and did pass just a secret_key)

Fixes #48759

(See also #49028 though)