update release script to be poetry-aware

[DMRobertson]

Follow-on from #11537.

As #12337 (comment) notes, the release script will need to be updated to edit pyproject.toml rather than synapse/init.py.

[clokep]

I think this was done in 730fcda. I'm going to close this, but please reopen if there's more to do @DMRobertson

[DMRobertson]

There was also a brief follow-up: a54d9b0. But yes, this is done now, thanks!