bump required minimum go version to 1.12.1 (strings package compatibility)

Author: Han Kang

hack/lib/golang.sh

@@ -382,7 +382,7 @@ EOF
   local go_version
   IFS=" " read -ra go_version <<< "$(go version)"
   local minimum_go_version
-  minimum_go_version=go1.11.1
+  minimum_go_version=go1.12.1
   if [[ "${minimum_go_version}" != $(echo -e "${minimum_go_version}\n${go_version[2]}" | sort -s -t. -k 1,1 -k 2,2n -k 3,3n | head -n1) && "${go_version[2]}" != "devel" ]]; then
     kube::log::usage_from_stdin <<EOF
 Detected go version: ${go_version[*]}.