go.mod: update dbft version (2.x)

[fyrchik]

Pull changes from #1667 , add tests for consensus.
Waiting for nspcc-dev/dbft#50 to settle.

[codecov]

Codecov Report

Merging #1675 (765568b) into master-2.x (89fb02a) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@              Coverage Diff               @@
##           master-2.x    #1675      +/-   ##
==============================================
- Coverage       65.62%   65.61%   -0.02%     
==============================================
  Files             161      161              
  Lines           14383    14395      +12     
==============================================
+ Hits             9439     9445       +6     
- Misses           4383     4392       +9     
+ Partials          561      558       -3     

Impacted Files	Coverage Δ
pkg/consensus/consensus.go	41.53% <100.00%> (-0.32%)	⬇️
pkg/rpc/client/wsclient.go	83.96% <0.00%> (+1.52%)	⬆️

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[roman-khimov]

Let's not touch it until there is a good reason to.