chore: bump toolchain to v4.18.0 (#1184)

Co-authored-by: Johan Commelin <johan@commelin.net>
Co-authored-by: leanprover-community-mathlib4-bot <leanprover-community-mathlib4-bot@users.noreply.github.com>
Co-authored-by: Matthew Ballard <matt@mrb.email>
Co-authored-by: Markus Himmel <markus@lean-fro.org>
Co-authored-by: Kyle Miller <kmill31415@gmail.com>
Co-authored-by: Mario Carneiro <di.gama@gmail.com>

Author: 7 people

lean-toolchain

@@ -1 +1 @@
-leanprover/lean4:v4.18.0-rc1
+leanprover/lean4:v4.18.0