all: use math/rand/v2 more

Updates #11058

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

net/netcheck/netcheck.go

@@ -14,7 +14,7 @@ import (
 	"io"
 	"log"
 	"maps"
-	"math/rand"
+	"math/rand/v2"
 	"net"
 	"net/http"
 	"net/netip"
@@ -1010,7 +1010,7 @@ func (c *Client) checkCaptivePortal(ctx context.Context, dm *tailcfg.DERPMap, pr
 		if len(rids) == 0 {
 			return false, nil
 		}
-		preferredDERP = rids[rand.Intn(len(rids))]
+		preferredDERP = rids[rand.IntN(len(rids))]
 	}
 
 	node := dm.Regions[preferredDERP].Nodes[0]

net/portmapper/upnp.go

@@ -15,7 +15,7 @@ import (
 	"encoding/xml"
 	"fmt"
 	"io"
-	"math/rand"
+	"math/rand/v2"
 	"net"
 	"net/http"
 	"net/netip"
@@ -165,7 +165,7 @@ func addAnyPortMapping(
 	// number in [0, 65535 - 1024] and then adding 1024 to it, shifting the
 	// range to [1024, 65535].
 	if externalPort < 1024 {
-		externalPort = uint16(rand.Intn(65535-1024) + 1024)
+		externalPort = uint16(rand.N(65535-1024) + 1024)
 	}
 
 	// First off, try using AddAnyPortMapping; if there's a conflict, the