go/upgrade/api: simplify upgrade descriptor

[ptrus]

No description provided.

[codecov]

Codecov Report

Merging #3793 (461889e) into master (a39d1e1) will increase coverage by 0.15%.
The diff coverage is 62.85%.

@@            Coverage Diff             @@
##           master    #3793      +/-   ##
==========================================
+ Coverage   67.20%   67.35%   +0.15%     
==========================================
  Files         404      404              
  Lines       40882    40851      -31     
==========================================
+ Hits        27473    27515      +42     
+ Misses       9522     9468      -54     
+ Partials     3887     3868      -19     

Impacted Files	Coverage Δ
go/common/version/version.go	80.48% <0.00%> (-0.91%)	⬇️
go/governance/api/sanity_check.go	88.73% <0.00%> (ø)
go/oasis-node/cmd/common/common.go	48.78% <50.00%> (-0.82%)	⬇️
go/oasis-node/cmd/root.go	85.71% <50.00%> (-2.53%)	⬇️
go/upgrade/upgrade.go	59.31% <60.00%> (ø)
go/upgrade/api/api.go	87.50% <80.00%> (-5.73%)	⬇️
go/governance/tests/tester.go	86.61% <83.33%> (-0.06%)	⬇️
go/upgrade/migrations/dummy.go	70.00% <100.00%> (ø)
go/consensus/tendermint/full/services.go	80.00% <0.00%> (-4.17%)	⬇️
go/consensus/tendermint/apps/beacon/state/state.go	68.25% <0.00%> (-3.18%)	⬇️
... and 31 more

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