Remove old build tag comments

Go is transitioning from the old '// +build' form to '//go:build', this
removes all uses of the old form, adding the new one where needed.

See https://go.googlesource.com/proposal/+/master/design/draft-gobuild.md

internal/tools.go

@@ -1,4 +1,4 @@
-// +build tools
+//go:build tools
 
 // This file is here to just explicitly tell `go mod vendor` that we depend
 // on oapi-codegen. Without this file, `go generate ./...` in Go >= 1.14 gets