Update CI tooling

[fasmat]

This updates gotestsum, staticcheck and golangci-lint to their newest versions and fixes complaints.

[codecov]

Codecov Report

Merging #319 (3e174ae) into develop (8f59581) will increase coverage by 0.2%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##           develop    #319     +/-   ##
=========================================
+ Coverage     72.7%   72.9%   +0.2%     
=========================================
  Files           17      17             
  Lines         1493    1493             
=========================================
+ Hits          1086    1089      +3     
+ Misses         300     298      -2     
+ Partials       107     106      -1     

Impacted Files	Coverage Δ
service/round.go	73.2% <100.0%> (ø)
service/service.go	78.0% <100.0%> (ø)

... and 1 file with indirect coverage changes