Remove support for python 3.8 (#17908)

### Pull Request Checklist

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* [X] Pull request is based on the develop branch
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- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
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* [X] [Code
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---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

docs/development/contributing_guide.md

@@ -322,7 +322,7 @@ The following command will let you run the integration test with the most common
 configuration:
 
 ```sh
-$ docker run --rm -it -v /path/where/you/have/cloned/the/repository\:/src:ro -v /path/to/where/you/want/logs\:/logs matrixdotorg/sytest-synapse:focal
+$ docker run --rm -it -v /path/where/you/have/cloned/the/repository\:/src:ro -v /path/to/where/you/want/logs\:/logs matrixdotorg/sytest-synapse:bullseye
 ```
 (Note that the paths must be full paths! You could also write `$(realpath relative/path)` if needed.)