Remove support for providing output format via `format` option (#7984)
See the provided breaking changes note for details.
Removes support for the deprecated `--format`option in the `ruff check`
CLI, `format` inference as `output-format` in the configuration file,
and the `RUFF_FORMAT` environment variable.
The error message for use of `format` in the configuration file could be
better, but would require some awkward serde wrappers and it seems hard
to present the correct schema to the user still.