Discussion: consider switching to the `--all-extras` flag provided by poetry 1.2+

We should consider moving away from the magic `pip install matrix-synapse[all]` invocation in favour of poetry's [`--all-extras`](https://python-poetry.org/docs/cli#install) flag. To support the current implementation, we have to duplicate our dependency list in `pyproject.toml`:

https://github.com/element-hq/synapse/blob/1920dfff40ad1078071e099a2afbfa31a5409e6b/pyproject.toml#L281-L316

Concerns:

* Old documentation suggesting `matrix-synapse[all]` would become invalid (unless we can provide some sort of shim, or warning when this is used?)
* Would this effectively require people use `poetry` over bare `pip`?

Finally, we should consider who actually needs to use a flag like `--all-extras`. I expect it'd only be Synapse developers and those looking for a quick command to prep the environment before running a test suite. If that's the case, perhaps it's not worth providing much of a transition from `matrix-synapse[all]` -> `--all-extras`.