DEV: Remove support for FA4 icon names (PR #9806)

This PR removes support for FontAwesome 4 icon names. It includes a migration for badge and group icons.

Should be considered for merge only after UX: use "icon-picker" & "image-uploader" fields to set group flair. by vinothkannans · Pull Request #9779 · discourse/discourse · GitHub is merged, because of column changes to group icons in that PR.