Updates the Stan submodule to 066164d3e.

stan-buildbot · stan-buildbot · commit 073d00529500 · 2025-07-01T10:28:55.000-04:00
diff --git a/StanHeaders/inst/include/upstream b/StanHeaders/inst/include/upstream
@@ -1 +1 @@
-Subproject commit f8e696908790cb6719c3824e00fa1d7aa46a0581
+Subproject commit 066164d3e3692a2852540fb87da50f2b9578b735
