ramips: Switch default kernel to 5.15

ramips: Switch default kernel to 5.15

I tested kernel 5.15 on my device for several times without any problems.
In my tests, 5.15 kernel has performance improvements such MGLRU.
Finally, initial kernel 6.1 support is imminent. All ramips subtargets have
5.15 as testing kernel. So, it's time to change.

Tested on my Archer C6 v3.2 (mt7621)

Signed-off-by: Rodrigo B. de Sousa Martins <rodrigo.sousa.577@gmail.com>
[reformat commit subject and message]
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>