Sentry noise: `Sentry is attempting to send N pending error messages`

This issue has been migrated from [#14069](https://github.com/matrix-org/synapse/issues/14069).

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E.g. https://sentry.tools.element.io/organizations/element/issues/608

```
2022-09-28 10:27:50,446 - twisted - 274 - INFO - sentinel - Main loop terminated.
2022-09-28 10:27:50,549 - twisted - 274 - ERROR - sentinel - Sentry is attempting to send 94 pending error messages
2022-09-28 10:27:50,555 - twisted - 274 - ERROR - sentinel - Waiting up to 2 seconds
2022-09-28 10:27:50,563 - twisted - 274 - ERROR - sentinel - Press Ctrl-C to quit
```

Presumably comes from https://github.com/getsentry/sentry-python/blob/4c09f3203d6d19789c6fa729a2e46557ad4ea913/sentry_sdk/integrations/atexit.py#L30-L33

Given that we redirect stderr -> twisted -> stdlib logging (for Reasons :TM:) this ends up as an ERROR; I think a warning makes more sense. There are some notes [here](https://docs.sentry.io/platforms/python/guides/flask/configuration/integrations/default-integrations/#atexit).

Start with the referenced sentry-python sentry_sdk/integrations/atexit.py lines and trace how Synapse redirects stderr through Twisted to standard logging. Confirm where the pending-message output is assigned ERROR, then identify whether the change belongs in Synapse or the Sentry integration. Done means this message is emitted as a warning rather than an ERROR.

Written by the indexing model from the issue text.