go: Start using new protobuf module location

[kostko]

The previous location has been deprecated.

[codecov]

Codecov Report

Merging #2867 into master will increase coverage by 0.03%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #2867      +/-   ##
==========================================
+ Coverage   67.57%   67.61%   +0.03%     
==========================================
  Files         344      344              
  Lines       33426    33511      +85     
==========================================
+ Hits        22589    22658      +69     
- Misses       7930     7934       +4     
- Partials     2907     2919      +12     

Impacted Files	Coverage Δ
go/common/sgx/aesm/aesm.go	63.63% <ø> (ø)
go/storage/mkvs/checkpoint/checkpoint.go	100.00% <ø> (ø)
go/storage/mkvs/checkpoint/checkpointer.go	65.18% <65.18%> (ø)
go/storage/mkvs/checkpoint/chunk.go	65.62% <81.81%> (+9.44%)	⬆️
go/worker/storage/committee/node.go	79.15% <83.33%> (-0.07%)	⬇️
go/storage/mkvs/checkpoint/restorer.go	100.00% <100.00%> (ø)
go/worker/storage/worker.go	74.82% <100.00%> (ø)
go/worker/storage/service_external.go	47.31% <0.00%> (-4.31%)	⬇️
go/consensus/tendermint/apps/staking/state/gas.go	77.08% <0.00%> (-4.17%)	⬇️
go/storage/metrics.go	78.26% <0.00%> (-3.27%)	⬇️
... and 25 more

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