Phase out support for go version 1.15 because current ginko is not backward compatible

[ccojocar]

We are going to support only the stable versions provided by the Go team.

[codecov-commenter]

Codecov Report

Merging #710 (beb67db) into master (c59cd6b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #710      +/-   ##
==========================================
+ Coverage   69.58%   69.60%   +0.02%     
==========================================
  Files          47       47              
  Lines        2857     2859       +2     
==========================================
+ Hits         1988     1990       +2     
  Misses        813      813              
  Partials       56       56              

Impacted Files	Coverage Δ
rules/errors.go	95.71% <100.00%> (+0.06%)	⬆️
rules/readfile.go	95.71% <100.00%> (+0.06%)	⬆️

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