ref: Deprecate `ListNewBlobs()` RPC

Patrick Steinhardtrequested to merge
pks-ref-deprecate-list-new-blobs into master

The ListNewBlobs() RPC call has been replaced upstream by the more flexible alternative ListBlobs(), which can do the same but across multiple new refs as once. Let's deprecate this RPC such that we can remove it in v14.4.

Note: the RPC isn't used in Rails anymore, but it's wrapper still exists. I'm removing it in gitlab!70363 (merged) though.

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