fix: pop Create Group screen when navigating to new Marmot group

After creating a Marmot group, the Create Group screen stays on the
back stack, so pressing back from the new group chat returns to the
empty creation form instead of the Messages screen. Swap `nav.nav`
for `nav.popUpTo(..., Route.CreateMarmotGroup::class)` so the creation
screen is removed inclusively as we navigate into the group.

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/marmotGroup/CreateGroupScreen.kt

@@ -80,7 +80,7 @@ fun CreateGroupScreen(
                     name = groupName.trim(),
                     description = "",
                 )
-                nav.nav(Route.MarmotGroupChat(nostrGroupId))
+                nav.popUpTo(Route.MarmotGroupChat(nostrGroupId), Route.CreateMarmotGroup::class)
             } catch (e: Exception) {
                 isCreating = false
                 launch(Dispatchers.Main) {