Set linux-cloud-tools version to 6.8 in expectation

Author: s4heid

bosh-stemcell/spec/assets/dpkg-list-ubuntu-azure-additions.txt

@@ -1,7 +1,7 @@
 azure-vm-utils
 cloud-init
-linux-cloud-tools-5.15
-linux-cloud-tools-5.15-generic
+linux-cloud-tools-6.8
+linux-cloud-tools-6.8-generic
 linux-cloud-tools-common
 linux-cloud-tools-generic
 netplan.io