chore: update toolchain

hargoniX · hargoniX · commit e6c7eafbb83c · 2025-06-06T10:24:50.000+02:00
diff --git a/lean-toolchain b/lean-toolchain
@@ -1 +1 @@
-leanprover/lean4:nightly-2025-05-27
+leanprover/lean4:v4.21.0-rc3
