Updates the Stan submodule to 11d1639ff.

stan-buildbot · stan-buildbot · commit dcc91987aba2 · 2026-03-19T11:35:14.000-04:00
diff --git a/StanHeaders/inst/include/upstream b/StanHeaders/inst/include/upstream
@@ -1 +1 @@
-Subproject commit 4786757adb2d09cf8c2a834da001bc1a515f3c96
+Subproject commit 11d1639ff63a64121351e92830e43f246425ab49
