doc,yocto: Upgrade to Yocto scarthgap release

Update all Kas files to use Yocto scarthgap or kronos-scarthgap.

Update yocto_version in the documentation.

Update LAYERSERIES_COMPAT_meta-kronos in layer.conf to scarthgap.

Signed-off-by: Peter Hoyes <peter.hoyes@arm.com>