Rename h_stack and v_stack to h_flex and v_flex, respectively (#4053)

This PR renames the `h_stack` and `v_stack` to `h_flex` and `v_flex`,
respectively.

We were previously using `h_stack` and `v_stack` to match SwiftUI, but
`h_flex` and `v_flex` fit better with the web/flexbox terminology that
the rest of GPUI uses.

Additionally, we were already calling the utility functions used to
implement `h_stack` and `v_stack` by the new names.

Release Notes:

- N/A

crates/auto_update/src/update_notification.rs

@@ -4,7 +4,7 @@ use gpui::{
 };
 use menu::Cancel;
 use util::channel::ReleaseChannel;
-use workspace::ui::{h_stack, v_stack, Icon, IconName, Label, StyledExt};
+use workspace::ui::{h_flex, v_flex, Icon, IconName, Label, StyledExt};
 
 pub struct UpdateNotification {
     version: SemanticVersion,
@@ -16,12 +16,12 @@ impl Render for UpdateNotification {
     fn render(&mut self, cx: &mut gpui::ViewContext<Self>) -> impl IntoElement {
         let app_name = cx.global::<ReleaseChannel>().display_name();
 
-        v_stack()
+        v_flex()
             .on_action(cx.listener(UpdateNotification::dismiss))
             .elevation_3(cx)
             .p_4()
             .child(
-                h_stack()
+                h_flex()
                     .justify_between()
                     .child(Label::new(format!(
                         "Updated to {app_name} {}",