gpui: Rename GPUSpecs to GpuSpecs (#22166)

This PR renames the `GPUSpecs` type to `GpuSpecs` to match Rust naming
conventions.

Release Notes:

- N/A

crates/gpui/src/platform/mac/window.rs

@@ -1102,7 +1102,7 @@ impl PlatformWindow for MacWindow {
         self.0.lock().renderer.sprite_atlas().clone()
     }
 
-    fn gpu_specs(&self) -> Option<crate::GPUSpecs> {
+    fn gpu_specs(&self) -> Option<crate::GpuSpecs> {
         None
     }