internal/gopathwalk: use os.ReadFile instead of ioutil.ReadFile

ioutil.ReadFile is deprecated.

Change-Id: I6854af32d05e4a3ea2ad00002fb39f4d87fc1794
Reviewed-on: https://go-review.googlesource.com/c/tools/+/508504
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/gopathwalk/walk.go

@@ -10,7 +10,6 @@ import (
 	"bufio"
 	"bytes"
 	"fmt"
-	"io/ioutil"
 	"log"
 	"os"
 	"path/filepath"
@@ -135,7 +134,7 @@ func (w *walker) init() {
 // The provided path is one of the $GOPATH entries with "src" appended.
 func (w *walker) getIgnoredDirs(path string) []string {
 	file := filepath.Join(path, ".goimportsignore")
-	slurp, err := ioutil.ReadFile(file)
+	slurp, err := os.ReadFile(file)
 	if w.opts.Logf != nil {
 		if err != nil {
 			w.opts.Logf("%v", err)