CI: Azure: Move to Ubuntu 22.04 image

As per actions/runner-images#6002 the Ubuntu
18.04 image is deprecated and will be removed by December 1, 2022.
Move to the Ubuntu 22.04 image as our base for launching our containers
from.

Signed-off-by: Tom Rini <trini@konsulko.com>

.azure-pipelines.yml

@@ -1,6 +1,6 @@
 variables:
   windows_vm: windows-2019
-  ubuntu_vm: ubuntu-18.04
+  ubuntu_vm: ubuntu-22.04
   macos_vm: macOS-10.15
   ci_runner_image: trini/u-boot-gitlab-ci-runner:focal-20220302-15Mar2022
   # Add '-u 0' options for Azure pipelines, otherwise we get "permission