Raise poetry-core version cap to 2.1.3 (#18575)

Request to raise the defensive version cap for poetry-core from 1.9.1 to
2.1.3.

My understanding is that the major version bump of poetry signals the
transition to standardized pyproject.toml metadata, but does not affect
backwards compatibility.

This is a subset of the changes in #18432

Fixes #18200

### Pull Request Checklist

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* [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).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

changelog.d/18575.misc

@@ -0,0 +1 @@
+Raise poetry-core version cap to 2.1.3.

pyproject.toml

@@ -374,7 +374,7 @@ tomli = ">=1.2.3"
 # runtime errors caused by build system changes.
 # We are happy to raise these upper bounds upon request,
 # provided we check that it's safe to do so (i.e. that CI passes).
-requires = ["poetry-core>=1.1.0,<=1.9.1", "setuptools_rust>=1.3,<=1.10.2"]
+requires = ["poetry-core>=1.1.0,<=2.1.3", "setuptools_rust>=1.3,<=1.10.2"]
 build-backend = "poetry.core.masonry.api"