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/debug_service.go b/metropolis/node/core/debug_service.go
index 814da9a..48a4c9b 100644
--- a/metropolis/node/core/debug_service.go
+++ b/metropolis/node/core/debug_service.go
@@ -20,7 +20,6 @@
 	"bufio"
 	"context"
 	"fmt"
-	"io/ioutil"
 	"os"
 	"regexp"
 	"strings"
@@ -215,7 +214,7 @@
 	if !safeTracingPropertyNamesRe.MatchString(name) {
 		return fmt.Errorf("disallowed tracing property name received: \"%v\"", name)
 	}
-	return ioutil.WriteFile("/sys/kernel/tracing/"+name, []byte(value+"\n"), 0)
+	return os.WriteFile("/sys/kernel/tracing/"+name, []byte(value+"\n"), 0)
 }
 
 func (s *debugService) Trace(req *apb.TraceRequest, srv apb.NodeDebugService_TraceServer) error {