linux: Implement local time zone support (#14610)

I decided to remove the GPUI APIs since `chrono` already provides this
functionality, and is already been used for this purpose in other parts
of the code (e.g.
[here](80402a6840/crates/zed/src/main.rs (L756))
or
[here](80402a6840/crates/ui/src/utils/format_distance.rs (L258)))

These usages end up calling the `time_format` crate, which takes in a
`UtcOffset`. It's probably cleaner to rewrite the crate to take in
`chrono` types, but that would require rewriting most of the code there.

Release Notes:

- linux: Use local time zone in chat and Git blame

crates/gpui/src/platform.rs

@@ -60,7 +60,6 @@ pub(crate) use mac::*;
 pub use semantic_version::SemanticVersion;
 #[cfg(any(test, feature = "test-support"))]
 pub(crate) use test::*;
-use time::UtcOffset;
 #[cfg(target_os = "windows")]
 pub(crate) use windows::*;
 
@@ -159,7 +158,6 @@ pub(crate) trait Platform: 'static {
         ""
     }
     fn app_path(&self) -> Result<PathBuf>;
-    fn local_timezone(&self) -> UtcOffset;
     fn path_for_auxiliary_executable(&self, name: &str) -> Result<PathBuf>;
 
     fn set_cursor_style(&self, style: CursorStyle);