Replace PREFIX/share/appdata with PREFIX/share/metainfo
As explained in https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#spec-component-location the use of PREFIX/share/appdata directory is deprecated and instead the path should be PREFIX/share/metainfo.
The desktop file name should also ideally be recoll.metainfo.xml, but this is not so critical and is hence kept unchanged by this commit.