go/common/version: Bump consensus protocol version to 2.0.0

[tjanez]

Since #3360 has been merged, we stated that from now onwards, only a change in a protocol's major version signifies a backward-incompatible change.

The change in #3458 is a breaking change, hence we should bump the consensus protocol's major version.

[codecov]

Codecov Report

Merging #3470 into stable/20.12.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@                Coverage Diff                 @@
##           stable/20.12.x    #3470      +/-   ##
==================================================
+ Coverage           65.60%   65.62%   +0.01%     
==================================================
  Files                 370      370              
  Lines               33349    33349              
==================================================
+ Hits                21880    21885       +5     
+ Misses               8262     8257       -5     
  Partials             3207     3207              

Impacted Files	Coverage Δ
go/common/version/version.go	78.94% <ø> (ø)
go/storage/mkvs/writelog/iterator.go	72.72% <0.00%> (-18.19%)	⬇️
go/common/grpc/errors.go	79.06% <0.00%> (-9.31%)	⬇️
go/worker/common/committee/runtime_host.go	62.88% <0.00%> (-5.16%)	⬇️
go/runtime/tagindexer/tagindexer.go	66.27% <0.00%> (-4.66%)	⬇️
go/storage/mkvs/iterator.go	79.13% <0.00%> (-4.32%)	⬇️
go/storage/mkvs/prefetch.go	54.71% <0.00%> (-3.78%)	⬇️
go/worker/storage/service_external.go	44.44% <0.00%> (-3.71%)	⬇️
...n/crypto/signature/signers/memory/memory_signer.go	72.41% <0.00%> (-3.45%)	⬇️
go/worker/storage/committee/node.go	71.42% <0.00%> (-2.86%)	⬇️
... and 29 more

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