go/backoff: Update cenkalti/backoff to 4.1.0

[ptrus]

Out upstream patch was accepted and released so the EnsurePermanent workaround can be simplified to only special case the context.Canceled method.

[codecov]

Codecov Report

Merging #3390 into master will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3390      +/-   ##
==========================================
- Coverage   66.15%   66.08%   -0.08%     
==========================================
  Files         371      371              
  Lines       33293    33285       -8     
==========================================
- Hits        22024    21995      -29     
- Misses       8039     8055      +16     
- Partials     3230     3235       +5     

Impacted Files	Coverage Δ
go/worker/common/p2p/error/error.go	100.00% <100.00%> (ø)
go/oasis-node/cmd/common/metrics/disk.go	65.38% <0.00%> (-19.24%)	⬇️
go/storage/mkvs/checkpoint/checkpointer.go	64.75% <0.00%> (-15.58%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	78.94% <0.00%> (-10.53%)	⬇️
go/staking/api/grpc.go	51.26% <0.00%> (-7.57%)	⬇️
go/consensus/tendermint/apps/staking/query.go	35.71% <0.00%> (-4.77%)	⬇️
go/runtime/tagindexer/tagindexer.go	66.27% <0.00%> (-4.66%)	⬇️
go/storage/api/metrics.go	86.15% <0.00%> (-4.62%)	⬇️
go/consensus/tendermint/abci/mux.go	57.67% <0.00%> (-3.94%)	⬇️
go/storage/api/root_cache.go	78.33% <0.00%> (-3.34%)	⬇️
... and 29 more

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