chore(*): Bump to Lean 3.43.0 (#14684)
Most of the changes in this upgrade are a consequence of https://github.com/leanprover-community/lean/pull/675, which removed almost all of `init/data/set.lean` from lean core so it could be migrated to mathlib. Other relevant core changes are https://github.com/leanprover-community/lean/pull/714, which removed a few order decidability instances, and https://github.com/leanprover-community/lean/pull/711, which caused a docstring to be rejected.
Co-authored-by: Yury G. Kudryashov <urkud@urkud.name>
Co-authored-by: Mauricio Collares <mauricio@collares.org>
Co-authored-by: Floris van Doorn <fpvdoorn@gmail.com>
