khepri_machine_v0: Fix spec of get_tree/1

[Why]
In `khepri_machine_v0`, it can only return a `tree_v0()` tree.

This was reported by Dialyzer in Erlang/OTP 28.

Author: dumbbell

src/khepri_machine_v0.erl

@@ -88,7 +88,7 @@ get_config(#khepri_machine{config = Config}) ->
 
 -spec get_tree(State) -> Tree when
       State :: khepri_machine_v0:state(),
-      Tree :: khepri_tree:tree().
+      Tree :: khepri_tree:tree_v0().
 %% @doc Returns the tree from the given state.
 %%
 %% @private