Prefer .metainfo.xml in AppData guidelines
Current AppStream documentation instructs saving all AppStream metainfo with
file extension metainfo.xml and mentions appdata.xml as historical (though
supported) format. Since metainfo.xml is now preferred, Fedora should use it
for all AppData.
Using only metainfo.xml extension also makes the guidelines somewhat simpler,
since less duplication is needed to cover two file extensions.