build: upgrade golangci-lint (#570)

Author: dajohi

.github/workflows/go.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Check out source
         uses: actions/checkout@v1
       - name: Install Linters
-        run: "curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $(go env GOPATH)/bin v1.18.0"
+        run: "curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $(go env GOPATH)/bin v1.21.0"
       - name: Build
         env:
           GO111MODULE: "on"