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/app.rs

@@ -14,7 +14,6 @@ use derive_more::{Deref, DerefMut};
 use futures::{channel::oneshot, future::LocalBoxFuture, Future};
 use slotmap::SlotMap;
 use smol::future::FutureExt;
-use time::UtcOffset;
 
 pub use async_context::*;
 use collections::{FxHashMap, FxHashSet, VecDeque};
@@ -647,11 +646,6 @@ impl AppContext {
         self.platform.restart(binary_path)
     }
 
-    /// Returns the local timezone at the platform level.
-    pub fn local_timezone(&self) -> UtcOffset {
-        self.platform.local_timezone()
-    }
-
     /// Updates the http client assigned to GPUI
     pub fn update_http_client(&mut self, new_client: Arc<dyn HttpClient>) {
         self.http_client = new_client;