Bump Synapse Dockerfile default to Python 3.12 (#17887)

changelog.d/17887.misc

@@ -0,0 +1 @@
+Bump the default Python version in the Synapse Dockerfile from 3.11 -> 3.12.

docker/Dockerfile

@@ -20,7 +20,7 @@
 # `poetry export | pip install -r /dev/stdin`, but beware: we have experienced bugs in
 # in `poetry export` in the past.
 
-ARG PYTHON_VERSION=3.11
+ARG PYTHON_VERSION=3.12
 
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 ### Stage 0: generate requirements.txt