feat: add os.ReadFile to G304 (#706)

In Go 1.16 or higher, the `io/ioutil` has been deprecated and the
`ioutil.ReadFile` function now calls `os.ReadFile`.

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

testutils/source.go

@@ -1787,6 +1787,22 @@ func main() {
 }`}, 1, gosec.NewConfig()}, {[]string{`
 package main
 
+import (
+"os"
+"log"
+)
+
+func main() {
+	f := os.Getenv("tainted_file")
+	body, err := os.ReadFile(f)
+	if err != nil {
+	log.Printf("Error: %v\n", err)
+	}
+	log.Print(body)
+
+}`}, 1, gosec.NewConfig()}, {[]string{`
+package main
+
 import (
 	"fmt"
 	"log"