ath79: switch to 5.15 as default kernel

ath79: switch to 5.15 as default kernel

The 5.15 kernel has new interesting features like MGLRU. Most of the
targets already have added support for testing kernel 5.15 since April
2022. Set 5.15 as default for all subtargets.

Testing support was added here:
- ae6bfb7d67c1 ("ath79: tiny: add 5.15 support for tiny subtarget")
- 9a0155bc4fa3 ("ath79: add 5.15 support for generic subtarget")
- 5af9aafabbc0 ("ath79: mikrotik: add 5.15 support for mikrotik subtarget")
- f3fa68e5153b ("ath79: nand: add 5.15 support for nand subtarget")

Tested on:
- Nanostation M5 XM (tiny)
- TP-Link EAP-225 Outdoor (generic)
- TP-Link CPE210 (generic)

Signed-off-by: Nick Hainke <vincent@systemli.org>