[RFC] Flag the package as abandoned on Packagist

With the introduction of native attributes to the PHP language, the Doctrine Annotations package has become legacy.

Since PHP attributes are an equal or better replacement in every aspect, we should take steps to flush out doctrine/annotations of the package ecosystem. The primary goal would be that no actively maintained composer package should list the `doctrine/annotations` package as a mandatory dependency in their latest release. Instead, PHP attributes should be offered as an alternative, so downstream projects can migrate at their own pace.

I wonder if flagging the package as abandoned would help in that regard. It would make Composer issue a warning when installing dependencies. Maintainers of PHP applications can check why `doctrine/annotations` is still in their dependencies.

* If it's being pulled by an upstream dependency, they could work with the maintainers of that dependency on making `doctrine/annotations` optional.
* If they themselves still use Doctrine Annotations, the warning would make them aware that there's a replacement.

Thoughts?