Use crypto/rand.Read instead of math/rand.Read

As of Go 1.20, `math/rand.Read` is deprecated. In addition to that, it also
isn't recommended for use in combination with anything cryptographic.

I haven't replaced all `math/rand` with `crypto/rand` imports because there
are still some legitimate uses for the `math/rand` package in some places.

Signed-off-by: Neil Twigg <neil@nats.io>

test/norace_test.go

@@ -18,9 +18,9 @@ package test
 
 import (
 	"context"
+	crand "crypto/rand"
 	"encoding/json"
 	"fmt"
-	"math/rand"
 	"net"
 	"net/url"
 	"os"
@@ -687,7 +687,7 @@ func TestNoRaceSlowProxy(t *testing.T) {
 	// Now test send BW.
 	const payloadSize = 64 * 1024
 	var payload [payloadSize]byte
-	rand.Read(payload[:])
+	crand.Read(payload[:])
 
 	// 5MB total.
 	bytesSent := (5 * 1024 * 1024)