grpclb: simplify stringifying of IPv6 with net.JoinHostPort (#8503)

This PR simplifies IP address handling in
`lbBalancer.processServerList`.

From [net.JoinHostPort](https://pkg.go.dev/net#JoinHostPort):

> JoinHostPort combines host and port into a network address of the form
"host:port". If host contains a colon, as found in literal IPv6
addresses, then JoinHostPort returns "[host]:port".

RELEASE NOTES: none

Author: alexandear

balancer/grpclb/grpclb_remote_balancer.go

@@ -82,14 +82,8 @@ func (lb *lbBalancer) processServerList(l *lbpb.ServerList) {
 		}
 
 		md := metadata.Pairs(lbTokenKey, s.LoadBalanceToken)
-		ip := net.IP(s.IpAddress)
-		ipStr := ip.String()
-		if ip.To4() == nil {
-			// Add square brackets to ipv6 addresses, otherwise net.Dial() and
-			// net.SplitHostPort() will return too many colons error.
-			ipStr = fmt.Sprintf("[%s]", ipStr)
-		}
-		addr := imetadata.Set(resolver.Address{Addr: fmt.Sprintf("%s:%d", ipStr, s.Port)}, md)
+		ipStr := net.IP(s.IpAddress).String()
+		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)
 		}