settings: Update MIME type for ostree repos
settings: Update MIME type for ostree repos

This MIME type was renamed in the process of being upstreamed; see
https://gitlab.freedesktop.org/xdg/shared-mime-info/merge_requests/22

Update it here so that upon insertion of USB drives containing OS/App
updates, GNOME Software will be automatically opened and the USB
category will be properly populated with apps.

https://phabricator.endlessm.com/T23512

endlessm/eos-theme