Updates the Stan submodule to 6168961a1.

stan-buildbot · stan-buildbot · commit 30b47f6ca95f · 2025-12-14T12:59:53.000-05:00
diff --git a/StanHeaders/inst/include/upstream b/StanHeaders/inst/include/upstream
@@ -1 +1 @@
-Subproject commit 4d65d4404fd460a2cfad3107990028567140002b
+Subproject commit 6168961a10ee401bee3067dc2d706f3da3f579ce
