Update Synapse Admin API quarantine path

[anoadragon453]

This path was deprecated nearly 4 years ago in matrix-org/synapse#6681.

[anoadragon453]

That test failure exists on develop as well, and isn't related to this PR. We should try to fix it separately, however we shouldn't merge this until that test passes, so that we can check this change works.

[anoadragon453]

The problem is that the Admin API request is going to the main process, whereas it should be going to the media worker. As we disable media functionality, including media-related Admin API functionality, in Sytest's Synapse when workers are enabled:

sytest/lib/SyTest/Homeserver/Synapse.pm

Line 265 in 7d3f92d

enable_media_repo => ( not $self->{workers} ),

https://github.com/element-hq/synapse/blob/1e68b56a623a0d7f98106482bfed223c22193050/synapse/rest/admin/__init__.py#L351-L352

[anoadragon453]

The problem is that the Admin API request is going to the main process, whereas it should be going to the media worker.

Should be fixed by #1381.