stop using math/rand.Seed and Read in tests, bump go.mod version to 1.20 (#3936)

frame_sorter_test.go

@@ -4,9 +4,10 @@ import (
 	"bytes"
 	"fmt"
 	"math"
-	"math/rand"
 	"time"
 
+	"golang.org/x/exp/rand"
+
 	"github.com/quic-go/quic-go/internal/protocol"
 
 	. "github.com/onsi/ginkgo/v2"
@@ -1408,7 +1409,7 @@ var _ = Describe("frame sorter", func() {
 				BeforeEach(func() {
 					seed := time.Now().UnixNano()
 					fmt.Fprintf(GinkgoWriter, "Seed: %d\n", seed)
-					rand.Seed(seed)
+					rand.Seed(uint64(seed))
 
 					callbacks = nil
 					dataLen = 25