all: replace internal rand shim with math/rand/v2

diff/fd/gradient_test.go

@@ -6,10 +6,10 @@ package fd
 
 import (
 	"math"
+	"math/rand/v2"
 	"testing"
 
 	"gonum.org/v1/gonum/floats"
-	"gonum.org/v1/gonum/internal/rand"
 )
 
 type Rosenbrock struct {
@@ -42,7 +42,7 @@ func (r Rosenbrock) FDf(x []float64, deriv []float64) (sum float64) {
 
 func TestGradient(t *testing.T) {
 	t.Parallel()
-	rnd := rand.New(rand.NewSource(1))
+	rnd := rand.New(rand.NewPCG(1, 1))
 	for i, test := range []struct {
 		nDim    int
 		tol     float64