refactor(ui): rename surface tokens to neutral background tokens

Replace --bui-surface-1..5 with --bui-bg-neutral-1..5 for both light
and dark themes. --bui-bg-neutral-1 replaces the deprecated --bui-bg-app,
and --bui-bg-neutral-2..5 extend the scale. The old bare --bui-bg-neutral-1..4
entries are removed from the deprecated section since their names are now
reused; the hover/pressed/disabled variants remain deprecated.

Updates colors.stories.tsx, the ESLint rule and tests, and all migration
documentation and the PR description accordingly.

Signed-off-by: Charles de Dreuille <charles.dedreuille@gmail.com>

.changeset/ui-semantic-color-tokens.md

@@ -8,15 +8,11 @@ The previous tokens remain in place for backward compatibility but are now depre
 
 **Migration:**
 
-**Surfaces**
+**Neutral backgrounds**
 
-| Deprecated           | Replacement       |
-| -------------------- | ----------------- |
-| `--bui-bg-app`       | `--bui-surface-1` |
-| `--bui-bg-neutral-1` | `--bui-surface-2` |
-| `--bui-bg-neutral-2` | `--bui-surface-3` |
-| `--bui-bg-neutral-3` | `--bui-surface-4` |
-| `--bui-bg-neutral-4` | `--bui-surface-5` |
+| Deprecated     | Replacement          |
+| -------------- | -------------------- |
+| `--bui-bg-app` | `--bui-bg-neutral-1` |
 
 **Foreground**