Mark drupal/legacy-project as abandoned

Problem/Motivation

No one should be using drupal/legacy-project - it is only used for the tarball. Maybe we can mark as abandoned in Drupal 11. Hopefully this wouldn't break packaging.

Proposed resolution

Use https://getcomposer.org/doc/04-schema.md#abandoned to recommend drupal/recommended-project instead.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet