Downgrade gunicorn for openfisca-core compatibility

pyproject.toml

@@ -16,7 +16,7 @@ python-dotenv = "^1.0.0"
 # openfisca-france = ">= 116.13.2"
 openfisca-france-reforms = { git = "https://git.leximpact.dev/leximpact/simulateur-socio-fiscal/openfisca/openfisca-france-reforms.git" }
 # openfisca-france-with-indirect-taxation = { git = "https://git.leximpact.dev/leximpact/simulateur-socio-fiscal/openfisca/openfisca-france-with-indirect-taxation.git" }
-gunicorn = "^23.0.0"
+gunicorn = "^21.0.0"
 pydantic-settings = "^2.1.0"
 
 [tool.poetry.dev-dependencies]