tree-wide: rewrite ioutil functions to their replacements

The ioutil package has been deprecated in Go 1.16 [1]. This CL removes
all our own users of that package and rewrites them to use their
replacements in the os package. I initially wanted to do this with a
gofix but because all replacements were signature-compatible I just
did it with a few string replaces and then ran goimports to fix up the
imports.

I intentionally didn't rewrite the patches as that would require a
different process and is IMO of less value.

[1] https://github.com/golang/go/issues/42026

Change-Id: Iac6663a1f1ee49f9b1c6e4b3d97e73f2c3b54a13
Reviewed-on: https://review.monogon.dev/c/monogon/+/449
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/metropolis/build/gotoolwrap/main.go b/metropolis/build/gotoolwrap/main.go
index eb70884..86dc3a1 100644
--- a/metropolis/build/gotoolwrap/main.go
+++ b/metropolis/build/gotoolwrap/main.go
@@ -36,7 +36,6 @@
 import (
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"log"
 	"os"
 	"os/exec"
@@ -81,7 +80,7 @@
 	// We list all files inside so that we can print them to the user for
 	// debugging purposes if that's not the case.
 	binFiles := make(map[string]bool)
-	files, err := ioutil.ReadDir(gorootBin)
+	files, err := os.ReadDir(gorootBin)
 	if err != nil {
 		log.Fatalf("Could not read dir $GOTOOLWRAP_GOROOT/bin (%q): %v", gorootBin, err)
 	}