all: use math/rand/v2 more

Updates #11058

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tstime/jitter.go

@@ -4,33 +4,10 @@
 package tstime
 
 import (
-	crand "crypto/rand"
-	"encoding/binary"
-	"math/rand"
-	"sync"
+	"math/rand/v2"
 	"time"
 )
 
-// crandSource is a rand.Source64 that gets its numbers from
-// crypto/rand.Reader.
-type crandSource struct{ sync.Mutex }
-
-var _ rand.Source64 = (*crandSource)(nil)
-
-func (s *crandSource) Int63() int64 { return int64(s.Uint64() >> 1) }
-
-func (s *crandSource) Uint64() uint64 {
-	s.Lock()
-	defer s.Unlock()
-	var buf [8]byte
-	crand.Read(buf[:])
-	return binary.BigEndian.Uint64(buf[:])
-}
-
-func (*crandSource) Seed(seed int64) {} // nope
-
-var durRand = rand.New(new(crandSource))
-
 // RandomDurationBetween returns a random duration in range [min,max).
 // If panics if max < min.
 func RandomDurationBetween(min, max time.Duration) time.Duration {
@@ -38,6 +15,5 @@ func RandomDurationBetween(min, max time.Duration) time.Duration {
 	if diff == 0 {
 		return min
 	}
-	ns := durRand.Int63n(int64(diff))
-	return min + time.Duration(ns)
+	return min + rand.N(max-min)
 }