Update mull-20

Author: philips-software-forest-releaser[bot]

.devcontainer/cpp/apt-requirements-clang.json

@@ -7,5 +7,5 @@
   "libclang-rt-20-dev": "1:20.1.8~++20250804090239+87f0227cb601-1~exp1~20250804210352.139",
   "lld-20": "1:20.1.8~++20250804090239+87f0227cb601-1~exp1~20250804210352.139",
   "llvm-20": "1:20.1.8~++20250804090239+87f0227cb601-1~exp1~20250804210352.139",
-  "mull-20": "0.30.0"
+  "mull-20": "0.31.1"
 }