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

Author: ffaf1

Cabal/ChangeLog.md

@@ -1,3 +1,6 @@
+# 3.12.0.0 [Francesco Ariis](mailto:[email protected]) March 2024
+* See https://github.com/haskell/cabal/blob/master/release-notes/Cabal-3.12.0.0.md
+
 # 3.10.3.0 [Hécate](mailto:[email protected]) January 2024
 * See https://github.com/haskell/cabal/blob/master/release-notes/Cabal-3.10.3.0.md