Enable clippy::map_flatten (#8733)

This PR enables the
[`clippy::map_flatten`](https://rust-lang.github.io/rust-clippy/master/index.html#/map_flatten)
rule and fixes the outstanding violations.

Release Notes:

- N/A

crates/project/src/project.rs

@@ -2775,7 +2775,7 @@ impl Project {
         let project_settings =
             ProjectSettings::get(Some((worktree_id.to_proto() as usize, Path::new(""))), cx);
         let lsp = project_settings.lsp.get(&adapter.name.0);
-        let override_options = lsp.map(|s| s.initialization_options.clone()).flatten();
+        let override_options = lsp.and_then(|s| s.initialization_options.clone());
 
         let server_id = pending_server.server_id;
         let container_dir = pending_server.container_dir.clone();