Downgrade 'Closing scope' error to a warning to prevent hammering sentry

[anoadragon453]

We noticed an error repeatedly coming up in Sentry after deploying e91a929 to matrix.org yesterday: https://sentry.tools.element.io/organizations/element/issues/725/ (internal link for those with access).

In summary, we're seeing hundreds of thousands of the following on incoming GET /_matrix/federation/v1/event/{eventId} requests:

Closing scope %s which is not the currently-active one %s
<synapse.logging.scopecontextmanager._LogContextScope object at 0x7f181f60e4f0>

@squahtx proposed that this was due to changes in #13489.

I imagine the messed up logging contexts have been there for a long time and it's only complaining extra hard now because of the new spans.

This PR reduces the error to a warning so that Sentry data is usable again. We should still separately fix the issue.

[babolivier]

lgtm as long as we commit to properly fixing it in the not too distant future.

[squahtx]

May be related to #13366 or #10855.

[richvdh]

oh man, I'm not very keen on this. can't we find the actual problem rather than just ignoring it?

[richvdh]

We've investigated further and found that #13499 is the cause of the problems here. We're investigating a fix, but otherwise we'll just revert #13499 for now.