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ARROW-14368: [CI] Use ubuntu-latest for Azure Pipelines

ubuntu-16.04 isn't available.

https://docs.microsoft.com/en-us/azure/devops/release-notes/2021/pipelines/sprint-193-update

> The removal of the image is planned for October 18th.

Closes #11453 from kou/ci-azure-ubuntu-16.04

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>