Revert urgency of notes endpoint to low

What does this MR do and why?

With notes using Action Cable, this is no longer an endpoint with a very high traffic share. So we revert it back to :low.

In #412823 (comment 1503833587), we can see that requests to this endpoint have been reduced significantly. Also, looking at Thanos, this endpoint now represents ~1.2% of traffic share which is down from ~8% noted in gitlab-com/gl-infra/scalability#2108 (closed).

(Feature flag is being removed in !129095 (merged))

This reverts part of !111202 (merged)

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