Add missing fa- icons for file_type_icon_class

This is a regression related to Fontawesome icon replacements which got introduced in !37922 (diffs). Some file icons that come from the file_type_icon_class helper are missing, e.g. see on https://gitlab.com/gitlab-org/gitlab/-/jobs/751254324/artifacts/browse/public/assets/

We need to re-add the following fa- icon classes to our fontawesome_custom.scss

• file-pdf-o
• file-image-o
• file-archive-o
• file-audio-o
• fa-file-video-o
• file-word-o
• file-excel-o
• file-powerpoint-o