golang: v↑ go1.16 (1.16.3 -> 1.16.4)

Going Go1.16.3 -> Go1.16.4 brings in fixes to compiler, runtime and stdlib
including security fix to net/http package:

https://golang.org/doc/devel/release.html#go1.16.minor

Tested on helloworld SR.

component/golang/buildout.cfg

@@ -53,8 +53,8 @@ environment-extra =
 
 [golang1.16]
 <= golang-common
-url = https://golang.org/dl/go1.16.3.src.tar.gz
-md5sum = 48183a40d6522f1ea59b7d63377b7072
+url = https://golang.org/dl/go1.16.4.src.tar.gz
+md5sum = 5884a4cb49067b9bcf6e69a1ef44259c
 
 # go1.16 needs go1.4 to bootstrap
 environment-extra =