build/debian: Bump kernel version to latest stable (6.1.123-1)

We verify that:
1. the ballooning patch introduced in aosp/3455686 still applies cleanly,
2. our script successfully builds the new kernel version for both x86 and arm.

Bug: None
Change-Id: If9ea54f89e507a2db17426f4f6105b8b273b14ec
Test: ./build_in_container.sh -k
Test: ./build_in_container.sh -a aarch64 -k
diff --git a/build/debian/build.sh b/build/debian/build.sh
index 1c7dd93..da01695 100755
--- a/build/debian/build.sh
+++ b/build/debian/build.sh
@@ -227,7 +227,11 @@
 
 	# NOTE: 6.1 is the latest LTS kernel for which Debian's kernel build scripts
 	#       work on Python 3.10, the default version on our Ubuntu 22.04 builders.
-	local debian_kver="6.1.119-1"
+	#
+	#       We track the latest Debian stable kernel version for the 6.1 branch,
+	#       which can be found at:
+	#       https://packages.debian.org/stable/linux-source-6.1
+	local debian_kver="6.1.123-1"
 
 	local dsc_file="linux_${debian_kver}.dsc"
 	local orig_ksrc_file="linux_${debian_kver%-*}.orig.tar.xz"