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/cluster/cluster.go b/metropolis/node/core/cluster/cluster.go
index aa293b0..60d7e15 100644
--- a/metropolis/node/core/cluster/cluster.go
+++ b/metropolis/node/core/cluster/cluster.go
@@ -29,7 +29,7 @@
 	"context"
 	"errors"
 	"fmt"
-	"io/ioutil"
+	"os"
 	"sync"
 
 	"google.golang.org/protobuf/proto"
@@ -129,7 +129,7 @@
 }
 
 func (m *Manager) nodeParamsFWCFG(ctx context.Context) (*apb.NodeParameters, error) {
-	bytes, err := ioutil.ReadFile("/sys/firmware/qemu_fw_cfg/by_name/dev.monogon.metropolis/parameters.pb/raw")
+	bytes, err := os.ReadFile("/sys/firmware/qemu_fw_cfg/by_name/dev.monogon.metropolis/parameters.pb/raw")
 	if err != nil {
 		return nil, fmt.Errorf("could not read firmware enrolment file: %w", err)
 	}