staticcheck: flag looking for "use of closed network connection" in error strings in Go 1.16+

[ainar-g]

Following golang/go#4373, programmers who want to check against errors about closed network connections should use errors.Is and not inspect the error text. So, in Go 1.16 and later this pattern (from the current code in golang.org/x/net/http2/server.go) should be flagged:

if strings.Contains(str, "use of closed network connection") {
	return true
}