ci: Do not test with GOPATH mode anymore

[tonyo]

Go Modules were first introduced in Go 1.11, and starting from Go 1.16 having GO111MODULE=on is the default behaviour.
Starting from Go 1.17, installing packages via go get (and, essentially, using the GOPATH mode) is basically deprecated, so there's no strong reason anymore for us to test with it anymore (we currently support Go 1.18+).

Closes #616

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.08 ⚠️

Comparison is base (231d5f0) 79.92% compared to head (0237d01) 79.84%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #618      +/-   ##
==========================================
- Coverage   79.92%   79.84%   -0.08%     
==========================================
  Files          38       38              
  Lines        3861     3861              
==========================================
- Hits         3086     3083       -3     
- Misses        669      671       +2     
- Partials      106      107       +1     

see 1 file with indirect coverage changes

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.