Convert blob service to use ListLFSPointers

What does this MR do?

Gitaly has recently deprecated both the GetLFSPointers and GetAllLFSPointers RPCs in favor of ListLFSPointers, which is a more flexible variant which simply takes a list of revisions (including the pseudo-revs --not and --all) and then returns all LFS pointers reachable via these revisions.

Convert the BlobService::get_new_lfs_pointers and BlobService::get_all_lfs_pointers RPCs to use the new RPC.

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