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/fileargs/fileargs.go b/metropolis/pkg/fileargs/fileargs.go
index bec8fca..88863da 100644
--- a/metropolis/pkg/fileargs/fileargs.go
+++ b/metropolis/pkg/fileargs/fileargs.go
@@ -21,7 +21,6 @@
 	"encoding/hex"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 
@@ -78,7 +77,7 @@
 
 	path := filepath.Join(f.path, name)
 
-	if err := ioutil.WriteFile(path, content, 0600); err != nil {
+	if err := os.WriteFile(path, content, 0600); err != nil {
 		f.lastError = err
 		return ""
 	}