refactor: move from io/ioutil to io and os packages (#5266)

The io/ioutil package has been deprecated as of Go 1.16 [1]. This commit
replaces the existing io/ioutil functions with their new definitions in
io and os packages.

[1]: https://golang.org/doc/go1.16#ioutil
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

tools/cmd/crd-doc-gen/main.go

@@ -18,7 +18,6 @@ package main
 
 import (
 	"flag"
-	"io/ioutil"
 	"log"
 	"os"
 	"path/filepath"
@@ -198,7 +197,7 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
-	err = ioutil.WriteFile(output, []byte(spec), 0644)
+	err = os.WriteFile(output, []byte(spec), 0644)
 	if err != nil {
 		log.Fatal(err)
 	}