vendor: update "grpc-gateway" to v1.4.0

[gyuho]

We forgot to update the actual Go dependencies from #9755.

[codecov-io]

Codecov Report

Merging #9756 into master will increase coverage by 0.61%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9756      +/-   ##
==========================================
+ Coverage   68.64%   69.26%   +0.61%     
==========================================
  Files         374      375       +1     
  Lines       35129    35179      +50     
==========================================
+ Hits        24114    24365     +251     
+ Misses       9242     9047     -195     
+ Partials     1773     1767       -6

Impacted Files	Coverage Δ
pkg/transport/timeout_conn.go	80% <0%> (-20%)	⬇️
proxy/grpcproxy/register.go	69.44% <0%> (-13.89%)	⬇️
pkg/adt/interval_tree.go	79.87% <0%> (-12.02%)	⬇️
etcdctl/ctlv3/command/lease_command.go	65.34% <0%> (-5.95%)	⬇️
integration/bridge.go	70.22% <0%> (-1.53%)	⬇️
lease/leasehttp/http.go	56.61% <0%> (-1.48%)	⬇️
clientv3/balancer/grpc1.7-health.go	84.88% <0%> (-1.46%)	⬇️
etcdctl/ctlv3/command/printer_simple.go	72.48% <0%> (-1.35%)	⬇️
etcdserver/api/v2http/client.go	85.51% <0%> (-1.21%)	⬇️
client/client.go	83.66% <0%> (-0.99%)	⬇️
... and 38 more

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[hexfusion]

@gyuho I see now. They cut 1.4.1 last night which has a feature that would allow us to output swagger to a single JSON file vs 3. grpc-ecosystem/grpc-gateway#658 If that seems reasonable I have cut a PR #9757.

[hexfusion]

lgtm

[hexfusion]

I am going to merge so the repo is stable we can look at 1.4.1 PR later.