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/fietsje/transitive.go b/build/fietsje/transitive.go
index 2e2a9f7..6ffd594 100644
--- a/build/fietsje/transitive.go
+++ b/build/fietsje/transitive.go
@@ -18,7 +18,6 @@
 
 import (
 	"fmt"
-	"io/ioutil"
 	"log"
 	"os"
 	"strings"
@@ -61,7 +60,7 @@
 
 	read := func(p string) []byte {
 		full := fmt.Sprintf("%s/%s", path, p)
-		data, err := ioutil.ReadFile(full)
+		data, err := os.ReadFile(full)
 		if err != nil {
 			panic(fmt.Sprintf("reading file %q: %v", full, err))
 		}