Move from Zed fonts to IBM Plex (#13596)

Release Notes:

- Changed the default fonts to IBM Plex Sans and IBM Plex Mono. If you'd
like to go back to using Zed Sans/Zed mono you need to
[download](https://github.com/zed-industries/zed-fonts/releases/tag/1.2.0)
them and install them.

crates/gpui/src/platform/cosmic_text/text_system.rs

@@ -205,7 +205,7 @@ impl CosmicTextSystemState {
     ) -> Result<SmallVec<[FontId; 4]>> {
         // TODO: Determine the proper system UI font.
         let name = if name == ".SystemUIFont" {
-            "Zed Sans"
+            "Zed Plex Sans"
         } else {
             name
         };