[Docs] Update spec decode + structured output in compat matrix (vllm-project#12373)

Signed-off-by: Russell Bryant <[email protected]>

Author: russellb

docs/source/features/compatibility_matrix.md

@@ -307,7 +307,7 @@ Check the '✗' with links to see tracking issue for unsupported feature/hardwar
      - ✅
      - ?
      - ?
-     - ✅
+     - [✗](gh-issue:11484)
      - ✅
      - ✗
      - ?