Bump version numbers for 3.12.0.0 (#9788)

* Bump version numbers for 3.12.0.0

Version fields, constraints, licences, and bootstrap.

☞ N.B.:
- This commit removes 8.10.7 from bootstrap list.
- This release only deals with Cabal, not cabal-install.

* Update index state in cabal.project.release

This is needed since we need the new (0.6.2.5) release of
hackage-security.

* Regenerate bootstrap .json files