all: partially migrate to math/rand/v2

This is not intended to be a completed transition since it leaves the
libraries unusable to external client code, but rather as a step towards
use of math/rand/v2. This initial step allows repair of sequence change
failures without having to worry about API difference.

internal/testrand/rand.go

@@ -8,7 +8,7 @@ package testrand
 import (
 	"flag"
 
-	"golang.org/x/exp/rand"
+	"gonum.org/v1/gonum/internal/rand"
 )
 
 var (