Updates the Stan submodule to 854756b20.

stan-buildbot · stan-buildbot · commit 247ab30b9dea · 2025-12-24T09:22:52.000-05:00
diff --git a/StanHeaders/inst/include/upstream b/StanHeaders/inst/include/upstream
@@ -1 +1 @@
-Subproject commit 5e5e5dc5863148baccf581d8a8f6ab044eb33828
+Subproject commit 854756b201a831a7b2cf0bb12ad77b08adb868a4
