Rename Max Mode to Burn Mode throughout code and docs (#31668)

Follow up to https://github.com/zed-industries/zed/pull/31470.

I started looking at config and changed preferred_completion_mode to
burn to only find its max so made changes to align it better with
rebrand. As this is in preview build now.

This doesn't touch zed_llm_client. Only the Zed changes the code and doc
to match the new UI of burn mode. There are still more things to be
renamed, though.

Release Notes:

- N/A

---------

Signed-off-by: Umesh Yadav <git@umesh.dev>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

crates/language_model/src/language_model.rs

@@ -242,7 +242,7 @@ pub trait LanguageModel: Send + Sync {
     /// Whether this model supports choosing which tool to use.
     fn supports_tool_choice(&self, choice: LanguageModelToolChoice) -> bool;
 
-    /// Returns whether this model supports "max mode";
+    /// Returns whether this model supports "burn mode";
     fn supports_max_mode(&self) -> bool {
         false
     }