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/node/core/mounts.go b/metropolis/node/core/mounts.go
index bb98462..a54331d 100644
--- a/metropolis/node/core/mounts.go
+++ b/metropolis/node/core/mounts.go
@@ -18,7 +18,6 @@
 
 import (
 	"fmt"
-	"io/ioutil"
 	"os"
 	"strings"
 
@@ -57,7 +56,7 @@
 	if err := unix.Mount("tmpfs", "/sys/fs/cgroup", "tmpfs", unix.MS_NOEXEC|unix.MS_NOSUID|unix.MS_NODEV, ""); err != nil {
 		panic(err)
 	}
-	cgroupsRaw, err := ioutil.ReadFile("/proc/cgroups")
+	cgroupsRaw, err := os.ReadFile("/proc/cgroups")
 	if err != nil {
 		panic(err)
 	}