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/test/platform.rs

@@ -257,10 +257,6 @@ impl Platform for TestPlatform {
         unimplemented!()
     }
 
-    fn local_timezone(&self) -> time::UtcOffset {
-        time::UtcOffset::UTC
-    }
-
     fn path_for_auxiliary_executable(&self, _name: &str) -> Result<std::path::PathBuf> {
         unimplemented!()
     }