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/kubernetes/provisioner.go b/metropolis/node/kubernetes/provisioner.go
index 42edf77..7288c84 100644
--- a/metropolis/node/kubernetes/provisioner.go
+++ b/metropolis/node/kubernetes/provisioner.go
@@ -20,7 +20,6 @@
 	"context"
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 
@@ -276,7 +275,7 @@
 		if err := os.Mkdir(volumePath, 0644); err != nil && !os.IsExist(err) {
 			return fmt.Errorf("failed to create volume directory: %w", err)
 		}
-		files, err := ioutil.ReadDir(volumePath)
+		files, err := os.ReadDir(volumePath)
 		if err != nil {
 			return fmt.Errorf("failed to list files in newly-created volume: %w", err)
 		}