Phase out support for go version 1.15 because current ginko is not backward compatible (#710)

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

Author: ccojocar

.github/workflows/ci.yml

@@ -21,7 +21,6 @@ jobs:
     strategy:
       matrix:
         go_version:
-          - '1.15'
           - '1.16'
           - '1.17'
     runs-on: ubuntu-latest