AppStream upstream metadata fails appstreamcli validation

[probonopd]

AppStream upstream metadata fails appstreamcli validation:

E - arduino.appdata.xml:<root>
    The metainfo file uses an ancient version of the AppStream specification, which can not be validated. Please migrate it to version 0.6 (or higher).

Validation failed.

(cc'ing @ximion fyi)

[cmaglie]

Should be solved with #5892