Fix logging context misuse when we fail to persist a federation event (#13089)

When we fail to persist a federation event, we kick off a task to remove
its push actions in the background, using the current logging context.
Since we don't `await` that task, we may finish our logging context
before the task finishes. There's no reason to not `await` the task, so
let's do that.

Signed-off-by: Sean Quah <seanq@matrix.org>