grpclb: replace net.IP with netip.Addr (#8918)

Contributes to #8884

This PR replaces deprecated `net.IP` usage with the modern `netip.Addr`
API in two packages:

- **grpclb**: Replace `net.IP(s.IpAddress).String()` with
`netip.AddrFromSlice()` in `processServerList`, adding proper error
handling for invalid IP addresses instead of silently producing `"?"`
from `net.IP.String()`.

Note: `x509.Certificate.IPAddresses` fields in test files
(`internal/credentials/xds/handshake_info_test.go`,
`security/advancedtls/crl_test.go`) are typed as `[]net.IP` by the
standard library, so those cannot be migrated. The xds/server and
xds/xdsclient packages are being addressed in #8909.

RELEASE NOTES: N/A

Author: veeceey

balancer/grpclb/grpclb_remote_balancer.go

@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"net/netip"
 	"sync"
 	"time"
 
@@ -82,7 +83,12 @@ func (lb *lbBalancer) processServerList(l *lbpb.ServerList) {
 		}
 
 		md := metadata.Pairs(lbTokenKey, s.LoadBalanceToken)
-		ipStr := net.IP(s.IpAddress).String()
+		var ipStr string
+		if ip, ok := netip.AddrFromSlice(s.IpAddress); ok {
+			ipStr = ip.String()
+		} else {
+			ipStr = fmt.Sprintf("? %x", s.IpAddress)
+		}
 		addr := imetadata.Set(resolver.Address{Addr: net.JoinHostPort(ipStr, fmt.Sprintf("%d", s.Port))}, md)
 		if lb.logger.V(2) {
 			lb.logger.Infof("Server list entry:|%d|, ipStr:|%s|, port:|%d|, load balancer token:|%v|", i, ipStr, s.Port, s.LoadBalanceToken)