[Doc] Update plugin deprecation notices (#33476)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

docs/design/plugin_system.md

@@ -154,4 +154,4 @@ The interface for the model/module may change during vLLM's development. If you
 !!! warning "Deprecations"
     - `use_v1` parameter in `Platform.get_attn_backend_cls` is deprecated. It has been removed in v0.13.0.
     - `_Backend` in `vllm.attention` is deprecated. It has been removed in v0.13.0. Please use `vllm.v1.attention.backends.registry.register_backend` to add new attention backend to `AttentionBackendEnum` instead.
-    - `seed_everything` platform interface is deprecated. It will be removed in v0.15.0 or later. Please use `vllm.utils.torch_utils.set_random_seed` instead.
+    - `seed_everything` platform interface is deprecated. It has been removed in v0.16.0. Please use `vllm.utils.torch_utils.set_random_seed` instead.