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/deps_monogon.go b/build/fietsje/deps_monogon.go
index 54e7d94..0abcd66 100644
--- a/build/fietsje/deps_monogon.go
+++ b/build/fietsje/deps_monogon.go
@@ -7,7 +7,7 @@
 import (
 	"bytes"
 	"fmt"
-	"io/ioutil"
+	"os"
 )
 
 // Monogon runs fietsje for all Monogon transitive dependencies.
@@ -134,7 +134,7 @@
 		return fmt.Errorf("could not render deps: %w", err)
 	}
 
-	err = ioutil.WriteFile(repositoriesBzlPath, buf.Bytes(), 0666)
+	err = os.WriteFile(repositoriesBzlPath, buf.Bytes(), 0666)
 	if err != nil {
 		return fmt.Errorf("could not write deps: %w", err)
 	}