chore(deprecation): retarget the 35 deprecations to 3.8.0 and add a usage scan (#681)

Moves the 35 @deprecated markers from 3.7.0 (already shipped with them in
place) to 3.8.0, and adds scripts/plugin_api_usage.py plus the generated
docs/DEPRECATIONS_3.8.md: who still calls or overrides each deprecated
method across core, the monorepo and third-party plugins. Removes nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/PLUGIN_DEVELOPMENT_GUIDE.md

@@ -520,14 +520,14 @@ When developing plugins, you'll need to use the APIs provided by the LEDMatrix s
   `display_manager.image` (a PIL Image) and call `update_display()`;
   there is no `draw_image()` helper method.
 - `draw_weather_icon()`, `draw_sun()`, `draw_cloud()` - Weather icons
-  (deprecated, removed in 3.7.0 — draw your own icons)
+  (deprecated, removed in 3.8.0 — draw your own icons)
 - `get_text_width()`, `get_font_height()` - Text utilities
 - `set_scrolling_state()`, `defer_update()` - Scrolling state management
 
 **Cache Manager** (`self.cache_manager`):
 - `get()`, `set()`, `delete()` - Basic caching
 - `get_cached_data_with_strategy()` - Advanced caching with strategies
-- `get_background_cached_data()` - deprecated, removed in 3.7.0 — use `get()`
+- `get_background_cached_data()` - deprecated, removed in 3.8.0 — use `get()`
 
 **Plugin Manager** (`self.plugin_manager`):
 - `get_plugin()`, `get_all_plugins()` - Access other plugins
@@ -535,7 +535,7 @@ When developing plugins, you'll need to use the APIs provided by the LEDMatrix s
 
 See [PLUGIN_API_REFERENCE.md](PLUGIN_API_REFERENCE.md) for complete
 documentation, and its [Deprecated APIs](PLUGIN_API_REFERENCE.md#deprecated-apis)
-table for everything removed in 3.7.0.
+table for everything removed in 3.8.0.
 
 ## 3rd Party Plugin Development