[6.x] Depreciate the elixir function

[GrahamCampbell]

This should give Laravel LTS and Laravel Stable users a heads up that this function will be removed in the next major upgrade they do.

[Vusys]

I think it's worth noting that while the method is called elixir, it's not really Laravel Elixir specific.

It simply reads the manifest created by gulp-rev and gets the versioned name of the file.

On an application I work on, we use Gulp to build our assets and gulp-rev for versioning/ cache busting. We never actually used Laravel Elixir. Just plain old Gulp, and this method works for gulp-rev, so we use it.

When it comes time to upgrade to Laravel 8, we will most likely just copy this method verbatim into my own helpers file and keep using it.