Add release notes for Cabal 3.12 (#9920)

* Add release notes for Cabal 3.12

Forwardport of #9785, #9908

Add changelog/release notes for Cabal 3.12 (#9785)

- generated with
     git log --pretty=oneline --no-color cabal-install-v3.10.1.0..HEAD > 3.12.prlog
- deduped
- stored WIP “release notes” file for future `cabal-install` release

3.12 changelogs updated (#9908)

* Update changelog for 3.12

(and for future WIP 3.12.1.0 cabal-install)

* Remove changelog.d files

* 3.12 changelog fixup (#9922)

* Incorporate Brandon’s suggestions

See #9920.

* Incorporate Artem’s suggestions

See #9920.

* Do not repeat yourself

* Fix whitespace

* Fix release notes grammar

See #9920.