realtek: switch to Kernel 5.15 by default

Getting ready for the next release.
Tested-by: NSander Vanheule <sander@svanheule.net>
Tested-by: NOlliver Schinagl <oliver@schinagl.nl>
Tested-by: NINAGAKI Hiroshi <musashino.open@gmail.com>
Signed-off-by: NPaul Spooren <mail@aparcar.org>

target/linux/realtek/Makefile

@@ -9,8 +9,7 @@ DEVICE_TYPE:=basic
 FEATURES:=ramdisk squashfs
 SUBTARGETS:=rtl838x rtl839x rtl930x rtl931x
 
-KERNEL_PATCHVER:=5.10
-KERNEL_TESTING_PATCHVER:=5.15
+KERNEL_PATCHVER:=5.15
 
 define Target/Description
 	Build firmware images for Realtek RTL83xx based boards.