Allow pip install to use setuptools_rust 1.6.0 (#15570)

* Allow `pip install` to use setuptools_rust 1.6.0

This was bumped by dependabot in #15512, but we didn't bump also raise
the version guard here. I don't know how we can avoid this happening in
the future.

Closes #15461.

Spotted in [1] by @landryb.

[1]: #15461 (comment)

* Changelog

Author: David Robertson

Diff for: changelog.d/15570.misc

@@ -0,0 +1 @@
+Allow `pip install` to use setuptools_rust 1.6.0 when building Synapse.

Diff for: pyproject.toml

@@ -368,7 +368,7 @@ furo = ">=2022.12.7,<2024.0.0"
 # 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>=1.2.0,<=1.5.0", "setuptools_rust>=1.3,<=1.5.2"]
+requires = ["poetry-core>=1.2.0,<=1.5.0", "setuptools_rust>=1.3,<=1.6.0"]
 build-backend = "poetry.core.masonry.api"