Fix `mage notice`

[belimawr]

The mage notice command has never been finalised (probably due to the lack of time), but we have a make rule that replaces it for the time being.

This issue is about using the implementation present on https://github.com/elastic/elastic-agent-libs to avoid code duplication and fixing all related issues.

One attempt has already been made on #1108, but due to time constraints, it was dropped in favour of the currently working make implementation.

[jlind23]

Mage notice has been removed in #1108 hence closing this.