fix: Avoid IPv6 in system DNS queries by default (zio#2280)

Currently, there are cases where users cannot use IPv6 due to their network
infrastructure not supporting it. Therefore, we should not resolve hostnames to
IPv6 addresses unless told otherwise.

This commit defaults DNS queries to IPv4-only. It also allows users to resolve
IPv6 addresses by setting the system property `java.net.preferIPv6Addresses`.

/claim zio#2280

Reference: https://docs.oracle.com/javase/8/docs/technotes/guides/net/ipv6_guide/

zio-http/src/main/scala/zio/http/DnsResolver.scala

@@ -16,8 +16,9 @@
 
 package zio.http
 
-import java.net.{InetAddress, UnknownHostException}
+import java.net.{InetAddress, Inet6Address, UnknownHostException}
 import java.time.Instant
+import java.lang.{System => JSystem}
 
 import zio._
 import zio.stacktracer.TracingImplicits.disableAutoTrace
@@ -27,15 +28,23 @@ trait DnsResolver {
 }
 
 object DnsResolver {
+  // NOTE: This system property is the only way for users to opt-in to IPv6.
+  // Even if the flag is set, we use both IPv4 and IPv6 and don't "prefer" IPv6
+  // in any way.
+  private final val useIPv6 = JSystem.getProperty("java.net.preferIPv6Addresses") == "true"
+
   def resolve(host: String)(implicit trace: Trace): ZIO[DnsResolver, UnknownHostException, Chunk[InetAddress]] =
     ZIO.serviceWithZIO(_.resolve(host))
 
   private final case class SystemResolver() extends DnsResolver {
-    override def resolve(host: String)(implicit trace: Trace): ZIO[Any, UnknownHostException, Chunk[InetAddress]] =
-      ZIO
-        .attemptBlocking(InetAddress.getAllByName(host))
-        .refineToOrDie[UnknownHostException]
-        .map(Chunk.fromArray)
+    override def resolve(host: String)(implicit trace: Trace): ZIO[Any, UnknownHostException, Chunk[InetAddress]] = {
+      for {
+        allHosts <- ZIO
+          .attemptBlocking(InetAddress.getAllByName(host))
+          .refineToOrDie[UnknownHostException]
+          .map(Chunk.fromArray)
+      } yield if (useIPv6) allHosts else allHosts.filter(!_.isInstanceOf[Inet6Address])
+    }
   }
 
   private[http] final case class CacheEntry(