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/pkg/loop/loop_test.go b/metropolis/pkg/loop/loop_test.go
index 16ead64..7f23f3e 100644
--- a/metropolis/pkg/loop/loop_test.go
+++ b/metropolis/pkg/loop/loop_test.go
@@ -19,7 +19,6 @@
 import (
 	"encoding/binary"
 	"io"
-	"io/ioutil"
 	"math"
 	"os"
 	"runtime"
@@ -36,7 +35,7 @@
 // bit unsigned integers) to detect offset correctness. File is always 128KiB
 // large (2^16 * 2 bytes).
 func makeTestFile() *os.File {
-	f, err := ioutil.TempFile("/tmp", "")
+	f, err := os.CreateTemp("/tmp", "")
 	if err != nil {
 		panic(err)
 	}
@@ -183,7 +182,7 @@
 	if os.Getenv("IN_KTEST") != "true" {
 		t.Skip("Not in ktest")
 	}
-	f, err := ioutil.TempFile("/tmp", "")
+	f, err := os.CreateTemp("/tmp", "")
 	assert.NoError(t, err)
 	empty1K := make([]byte, 1024)
 	for i := 0; i < 64; i++ {