Reloading the log config on SIGHUP writes every record back into
Twisted's logging. `redirect_stdio_to_logs()` replaces `sys.stderr` with
a `LoggingFile` after startup, and `_load_logging_config` then rebuilds
`StreamHandler`s on it, so records hit the `logging during logging`
guard in `one_time_logging_setup` and JSON lines get wrapped a second
time.
`dictConfig` now runs with `sys.__stdout__` and `sys.__stderr__` swapped
in, and the redirected streams are put back afterwards, including when
the config is invalid. `tests/config/test_logger.py` covers both. Not
covered: Windows and configs that use a `FileHandler`.
Fixes#20091
### Pull Request Checklist
* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (ruff check, ruff format and mypy run on the changed files)
