build: Bump plugin API version for recent GsApp API changes

So out of tree plugins can be parallel installed in a way that means
gnome-software only loads the ones which use a compatible
libgnomesoftware ABI.

Spotted by Phaedrus Leeds.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #1146
2 jobs for 1146-carousel-fixes in 4 minutes and 23 seconds (queued for 1 second)