Increasing request urgency for unleash features endpoint

What does this MR do and why?

Part of gitlab-com/gl-infra/scalability#2109 (closed).

Some time ago, due to redis-cache issue, the urgency was lowered until the issue could be addressed.

The endpoint currently has an urgency set to low (target duration 5s), but it's performing much better than that:

total	low (5s)	medium (0.5s)	default (1s)
641,327,177	1,590	175,032	36,929
	0.9999975208	0.9997270785	0.9999424178

(source: https://log.gprd.gitlab.net/goto/077bb3d0-b8f1-11ed-85ed-e7557b0a598c)

Therefore, the urgency can be tightened to be medium.

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