Set workflow default kernel to linux-image-arm64

Drop the default kernel specification from debos.yml. This causes the
default in the rootfs debos recipe to be used, which is
linux-image-arm64, to be fulfilled from debian-backports.

The previous kernel of linux-image-6.16.7-qcom1+ fails fastrpc_test, but
it does work from linux-image-arm64. Further, that kernel has fallen
behind because it required manual updates.

Signed-off-by: Robie Basak <robie.basak@oss.qualcomm.com>

Author: basak-qcom

.github/workflows/debos.yml

@@ -16,7 +16,7 @@ on:
         # the package name will be passed to APT which interprets the trailing
         # plus sign in the package name as a request to install the package, so
         # use two plus signs
-        default: linux-image-6.16.7-qcom1++
+        default: ''
       debos_extra_args:
         description: Extra arguments to pass to debos (e.g. -t dtb:qcom/some.dtb)
         type: string