Drop custom file upload plugin in favor of CSS solution

Created by: MartijnCuppens

CSS only alternative solution for https://github.com/twbs/bootstrap/pull/31085. Advantages:

• No js dependencies
• No need to add translated button labels
• Labels are automatically adjusted when with <input type="file" multiple>

TODO:

• Wait until legacy Edge is dropped This isn't really necessary, since legacy Edge already has a decent fallback
• Extend migration guide
• Check if the alpha version is correct
• Fix border radii on iOS

https://deploy-preview-31955--twbs-bootstrap.netlify.app/docs/5.0/forms/form-control/#file-input https://deploy-preview-31955--twbs-bootstrap.netlify.app/docs/5.0/forms/validation/#supported-elements

Closes #30265 (closed) Closes #31085