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.

graph/path/shortest_test.go

@@ -11,12 +11,11 @@ import (
 	"slices"
 	"testing"
 
-	"golang.org/x/exp/rand"
-
 	"gonum.org/v1/gonum/graph"
 	"gonum.org/v1/gonum/graph/graphs/gen"
 	"gonum.org/v1/gonum/graph/simple"
 	"gonum.org/v1/gonum/internal/order"
+	"gonum.org/v1/gonum/internal/rand"
 )
 
 var shortestTests = []struct {