overlays: README: Deprecate vc4-kms-kippah-7inch

The vc4-kms-kippah-7inch overlay has been replaced by the container
overlay vc4-kms-dpi-panel, using the "kippah-7inch" parameter. The
original overlay has been retained for now to avoid breaking
existing installations, but that doesn't make it any less deprecated,
so update the README accordingly.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>