Upgrade poetry-core range to fix issue with .so file (#16702)

poetry-core 1.8.x includes a fix which properly moves the generate
synapse_rust.abi3.so file to the synapse directory when using an
editable install.

Without this change developers are left with a confusing experience
of the synapse.synapse_rust module not being found after installation.

changelog.d/16702.misc

@@ -0,0 +1 @@
+Raise poetry-core upper bound to <=1.8.1. This allows contributors to import Synapse after `poetry install`ing with Poetry 1.6 and above. Contributed by Mo Balaa.

pyproject.toml

@@ -382,7 +382,7 @@ furo = ">=2022.12.7,<2024.0.0"
 # 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,<=1.7.0", "setuptools_rust>=1.3,<=1.8.1"]
+requires = ["poetry-core>=1.1.0,<=1.8.1", "setuptools_rust>=1.3,<=1.8.1"]
 build-backend = "poetry.core.masonry.api"