chore: update toolchain to first stable release

mhuisi · mhuisi · commit 2245a1dda89c · 2023-09-21T10:52:36.000+02:00
diff --git a/lean-toolchain b/lean-toolchain
@@ -1 +1 @@
-leanprover/lean4:nightly-2023-08-23
+leanprover/lean4:v4.0.0
