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.go b/metropolis/pkg/loop/loop.go
index c338f04..a4974a8 100644
--- a/metropolis/pkg/loop/loop.go
+++ b/metropolis/pkg/loop/loop.go
@@ -28,7 +28,6 @@
 import (
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"math/bits"
 	"os"
 	"sync"
@@ -223,7 +222,7 @@
 
 // BackingFilePath returns the path of the backing file
 func (d *Device) BackingFilePath() (string, error) {
-	backingFile, err := ioutil.ReadFile(fmt.Sprintf("/sys/block/loop%d/loop/backing_file", d.num))
+	backingFile, err := os.ReadFile(fmt.Sprintf("/sys/block/loop%d/loop/backing_file", d.num))
 	if err != nil {
 		return "", fmt.Errorf("failed to get backing file path: %w", err)
 	}