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/mac/platform.rs

@@ -49,7 +49,6 @@ use std::{
     slice, str,
     sync::Arc,
 };
-use time::UtcOffset;
 
 use super::renderer;
 
@@ -760,14 +759,6 @@ impl Platform for MacPlatform {
         }
     }
 
-    fn local_timezone(&self) -> UtcOffset {
-        unsafe {
-            let local_timezone: id = msg_send![class!(NSTimeZone), localTimeZone];
-            let seconds_from_gmt: NSInteger = msg_send![local_timezone, secondsFromGMT];
-            UtcOffset::from_whole_seconds(seconds_from_gmt.try_into().unwrap()).unwrap()
-        }
-    }
-
     fn path_for_auxiliary_executable(&self, name: &str) -> Result<PathBuf> {
         unsafe {
             let bundle: id = NSBundle::mainBundle();