tunnel: newIncompleteFuture requires API 31

Fixes: 3b6ab660 ("ui: raise min SDK to 24")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tunnel/src/main/java/com/wireguard/android/backend/GoBackend.java

@@ -404,7 +404,10 @@ public final class GoBackend implements Backend {
                     tunnel.onStateChange(State.DOWN);
                 }
             }
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S)
                 vpnService = vpnService.newIncompleteFuture();
+            else
+                vpnService = new CompletableFuture<>();
             super.onDestroy();
         }