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/pkg/tpm/tpm.go b/metropolis/pkg/tpm/tpm.go
index ab02dd3..f84aac6 100644
--- a/metropolis/pkg/tpm/tpm.go
+++ b/metropolis/pkg/tpm/tpm.go
@@ -24,7 +24,6 @@
 	"crypto/x509"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strconv"
@@ -584,5 +583,5 @@
 // result can be parsed by eventlog.  As this library currently doesn't support
 // extending PCRs it just returns the log as supplied by the EFI interface.
 func GetMeasurementLog() ([]byte, error) {
-	return ioutil.ReadFile("/sys/kernel/security/tpm0/binary_bios_measurements")
+	return os.ReadFile("/sys/kernel/security/tpm0/binary_bios_measurements")
 }