Update default kernel version to 6.6 and drop configs and patches for
kernel 6.1. We can also omit the conditional to include DTS dir.
Signed-off-by: Stefan Kalscheuer <stefan@stklcode.de>
Link: https://github.com/openwrt/openwrt/pull/15449
Signed-off-by: Robert Marko <robimarko@gmail.com>