poky: change preferred kernel version to 5.15 in poky-alt

5.10 has been removed from oe-core, we change the alt distro to
the 5.15 kernel.

(From meta-yocto rev: 7077d849ef8588b65a7a3ae61b49292407ac13d7)

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-poky/conf/distro/include/poky-distro-alt-test-config.inc

@@ -2,7 +2,7 @@
 DISTRO_FEATURES:append = " pam"
 
 # Use the LTSI Kernel
-PREFERRED_VERSION_linux-yocto = "5.10%"
+PREFERRED_VERSION_linux-yocto = "5.15%"
 
 # Ensure the kernel nfs server is enabled
 KERNEL_FEATURES:append:pn-linux-yocto = " features/nfsd/nfsd-enable.scc"