Updates the Stan submodule to 8a21c1287.

stan-buildbot · stan-buildbot · commit 84420afc7759 · 2025-04-25T16:00:21.000-04:00
diff --git a/stan b/stan
@@ -1 +1 @@
-Subproject commit 7339c22164396257482f8e4f581e600bcb03ab5d
+Subproject commit 8a21c1287e7754ab02b610ac63c5288f06f1406c
