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/logtree/unraw/unraw_test.go b/metropolis/pkg/logtree/unraw/unraw_test.go
index afd1da9..5526feb 100644
--- a/metropolis/pkg/logtree/unraw/unraw_test.go
+++ b/metropolis/pkg/logtree/unraw/unraw_test.go
@@ -4,7 +4,6 @@
 	"context"
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"os"
 	"syscall"
 	"testing"
@@ -21,7 +20,7 @@
 }
 
 func TestNamedPipeReader(t *testing.T) {
-	dir, err := ioutil.TempDir("/tmp", "metropolis-test-named-pipe-reader")
+	dir, err := os.MkdirTemp("/tmp", "metropolis-test-named-pipe-reader")
 	if err != nil {
 		t.Fatalf("could not create tempdir: %v", err)
 	}