chore: bump toolchain to v4.3.0-rc1 (leanprover-community#8051)

This incorporates changes from 

* leanprover-community#7845
* leanprover-community#7847
* leanprover-community#7853
* leanprover-community#7872 (was never actually made to work, but the diffs in `nightly-testing` are unexciting: we need to fully qualify a few names)

They can all be closed when this is merged.



Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
Co-authored-by: Eric Wieser <wieser.eric@gmail.com>

chore: bump toolchain to v4.2.0 (leanprover-community#8050)

Co-authored-by: Scott Morrison <scott.morrison@gmail.com>

chore: bump toolchain to v4.2.0-rc3

chore: bump toolchain to v4.2.0-rc2 (leanprover-community#7703)

This includes all the changes from leanprover-community#7606.



Co-authored-by: Scott Morrison <scott.morrison@gmail.com>

chore: bump toolchain to v4.2.0-rc1 (leanprover-community#7370)

This rolls in the changed from leanprover-community#6928.

Co-authored-by: Thomas Murrills <thomasmurrills@gmail.com>



Co-authored-by: Scott Morrison <scott.morrison@gmail.com>

chore: bump toolchain to v4.1.0 (leanprover-community#7369)

Co-authored-by: Scott Morrison <scott.morrison@gmail.com>

chore: bump to v4.1.0-rc1 (2nd attempt) (leanprover-community#7216)





Co-authored-by: Eric Wieser <wieser.eric@gmail.com>
Co-authored-by: Scott Morrison <scott.morrison@gmail.com>

chore: bump to Lean v4.0.0 (leanprover-community#7024)

Co-authored-by: Scott Morrison <scott.morrison@gmail.com>

chore: bump to v4.0.0-rc4 (leanprover-community#6873)





Co-authored-by: Scott Morrison <scott.morrison@gmail.com>

chore: bump to v4.0.0-rc2 (leanprover-community#6824)

This is actually a no-op, as `v4.0.0-rc2` points to the same commit as `nightly-2023-08-23`. But the plan is to wean us off the nightly releases and on to release candidates, so might as well start now.



Co-authored-by: Scott Morrison <scott.morrison@gmail.com>