Migrate io/ioutil uses to new APIs.

ioutil has been deprecated since Go 1.16. The functions were moved to
some combination of io and os. See https://pkg.go.dev/io/ioutil.

(File-related functions went to os. Generic things went to io. Names
were kept the same except TempDir and TempFile are os.MkdirTemp and
os.CreateTemp, respectively.)

Change-Id: I031306f69e70424841df08f64fa9d90f31780928
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/55186
Auto-Submit: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
diff --git a/util/fetch_ech_config_list.go b/util/fetch_ech_config_list.go
index badaae2..8f09e66 100644
--- a/util/fetch_ech_config_list.go
+++ b/util/fetch_ech_config_list.go
@@ -18,7 +18,6 @@
 	"errors"
 	"flag"
 	"fmt"
-	"io/ioutil"
 	"log"
 	"net"
 	"os"
@@ -379,7 +378,7 @@
 		}
 
 		outFile := path.Join(*outDir, fmt.Sprintf("ech-config-list-%d", echConfigListCount))
-		if err = ioutil.WriteFile(outFile, record.ech, 0644); err != nil {
+		if err = os.WriteFile(outFile, record.ech, 0644); err != nil {
 			log.Printf("Failed to write file: %s\n", err)
 			os.Exit(1)
 		}