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/build/static_binary_tarball/main.go b/build/static_binary_tarball/main.go
index 72ee5d0..b7b3612 100644
--- a/build/static_binary_tarball/main.go
+++ b/build/static_binary_tarball/main.go
@@ -20,7 +20,6 @@
 	"archive/tar"
 	"flag"
 	"io"
-	"io/ioutil"
 	"log"
 	"os"
 	"path"
@@ -39,7 +38,7 @@
 func main() {
 	flag.Parse()
 	var spec spec.Spec
-	specRaw, err := ioutil.ReadFile(*specPath)
+	specRaw, err := os.ReadFile(*specPath)
 	if err != nil {
 		log.Fatalf("failed to open spec file: %v", err)
 	}