Cherrypick: Update defaultbackend to v1.5

[bowei]

Update defaultbackend to 1.5: move /metrics to a private endpoint (:10254).

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[foxish]

@bowei - can you update the release note? @nicksardo, @zmerlynn PTAL for LGTM

[jonpulsifer]

👋 can we get this in? here's a stab at a release note

Update defaultbackend to 1.5: moved `/metrics` behind tcp:10254

[bowei]

sgtm

[bowei]

/retest