Update go versions in build matrix

These are the latest versions listed on https://go.dev/dl/

.github/workflows/main.yml

@@ -6,7 +6,7 @@ jobs:
  runs-on: ubuntu-latest
  strategy:
  matrix:
- go_version: ["1.17.6", "1.16.5", "1.15.13"]
+ go_version: ["1.17.8", "1.16.15", "1.15.15"]
  steps:
  - uses: actions/checkout@v2
  - name: Setup Go