grpcproxy: forward PrevKv flag in Put

[heyitsanthony]

/cc @visheshnp

[codecov-io]

Codecov Report

Merging #8366 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #8366      +/-   ##
==========================================
- Coverage   76.74%   76.71%   -0.04%     
==========================================
  Files         349      349              
  Lines       27366    27368       +2     
==========================================
- Hits        21003    20994       -9     
- Misses       4857     4868      +11     
  Partials     1506     1506

Impacted Files	Coverage Δ
proxy/grpcproxy/kv.go	99.25% <100%> (+0.01%)	⬆️
pkg/adt/interval_tree.go	82.58% <0%> (-8.11%)	⬇️
etcdserver/api/v3rpc/watch.go	89.3% <0%> (-4.19%)	⬇️
proxy/grpcproxy/watcher.go	85.71% <0%> (-4.09%)	⬇️
pkg/transport/listener_tls.go	65.56% <0%> (-0.67%)	⬇️
etcdserver/api/v3rpc/lease.go	96.42% <0%> (ø)	⬆️
clientv3/watch.go	95.04% <0%> (ø)	⬆️
lease/leasehttp/http.go	62.59% <0%> (ø)	⬆️
etcdserver/server.go	80.2% <0%> (+0.23%)	⬆️
etcdserver/apply.go	91.09% <0%> (+0.52%)	⬆️
... and 9 more

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[gyuho]

lgtm.