Updates the Stan submodule to 6f042983a.

stan-buildbot · stan-buildbot · commit ce1d881d1ca6 · 2025-09-30T15:42:21.000-04:00
diff --git a/StanHeaders/inst/include/upstream b/StanHeaders/inst/include/upstream
@@ -1 +1 @@
-Subproject commit 7a66541ef6ddcc250b94d7d90411f60d2f89640e
+Subproject commit 6f042983a3aab58481c4269301e548332ea4944d
