providers: Update disroot.org

disroot.org now specifies their upload limit and message retention duration (source):

• Users can upload files upto 512 MB
• Messages and files are stored up to 1 month

This is currently marked as "unspecified" on the service.

Also, user registration is allowed (though not through in-band registration). I clicked on the "user" icon in the toolbar and then "Register" and got a registration form. They sometimes pause registrations for a while, but I think this should upgrade them to "B"?