Remove legacy admonition keywords

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#issues).

### Motivation

We historically have admonition keyword "aliases" that we don't even document since v2.0.


Those came from an old v2 dependency: https://github.com/elviswolcott/remark-admonitions

In Docusaurus v4 is time to remove them officially and throw an error for sites that still use them.

Note: in Docusaurus v3 we made `:::warning` the official admonition, and `:::caution` an alias to remove in v4 (see https://github.com/facebook/docusaurus/issues/7558)

**EDIT**: maybe we should have some kind of `siteConfig.markdown.admonitions` config and probably let users define their own set of directive/icon/color/label without requiring swizzling anything?

### Self-service

- [X] I'd be willing to do some initial work on this proposal myself.