Updates the Stan submodule to 8437b5f.

stan-buildbot · stan-buildbot · commit d73ac7c131b0 · 2021-08-20T03:32:18.000-04:00
diff --git a/stan b/stan
@@ -1 +1 @@
-Subproject commit 04e5e83167fbf0d558030dffdb4849499d06f16c
+Subproject commit 8437b5f505af6961717399cff354d12fad330fc7
