Fix bug when broadcasting deprecated config

[MetRonnie]

Repro

Any simple workflow

$ cylc broadcast wflow -p 1 -n foo -s '[job]execution time limit'

Traceback (most recent call last):
  File ".../bin/cylc", line 9, in <module>
    sys.exit(main())
             ^^^^^^
  File ".../lib/python3.12/site-packages/cylc/flow/scripts/cylc.py", line 646, in main
    ret = _main(opts, cmd_args)
          ^^^^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.12/site-packages/cylc/flow/scripts/cylc.py", line 706, in _main
    return execute_cmd(command, *cmd_args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.12/site-packages/cylc/flow/scripts/cylc.py", line 329, in execute_cmd
    entry_point.load()(*args)
  File ".../lib/python3.12/site-packages/cylc/flow/terminal.py", line 298, in wrapper
    wrapped_function(*wrapped_args, **wrapped_kwargs)
  File ".../lib/python3.12/site-packages/cylc/flow/scripts/broadcast.py", line 506, in main
    rets = asyncio.run(_main(options, *ids))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.12/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File ".../lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.12/asyncio/base_events.py", line 691, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File ".../lib/python3.12/site-packages/cylc/flow/scripts/broadcast.py", line 511, in _main
    return await call_multi_async(
           ^^^^^^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.12/site-packages/cylc/flow/network/multi.py", line 139, in call_multi_async
    out, err, outcome = _process_response(
                        ^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.12/site-packages/cylc/flow/network/multi.py", line 227, in _process_response
    raise response
  File ".../lib/python3.12/site-packages/cylc/flow/async_util.py", line 436, in _inner
    return await coroutine(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../lib/python3.12/site-packages/cylc/flow/scripts/broadcast.py", line 476, in run
    modified_settings = result['response'][0]
                        ~~~~~~~~~~~~~~~~~~^^^
KeyError: 0

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] No dependency changes
• [x] Tests are included
• [ ] Changelog entry included if this is a change that can affect users
• [x] Docs not needed
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.