[DON'T MERGE] [SE-3817] fix: replace ubuntu xenial with focal

This PR ensures that the new app servers will run on Ubuntu/Focal instead of Ubuntu/Xenial. This PR can be merged anytime as the effective configuration is set on the OCIM instance and not read from the changed settings - we are not using the default value.

The changes introduced in https://github.com/edx/configuration/commit/7695e698fbb4dd2ff6c58a3163b73226ebef289e breaks the app server provisioning with Python 3.5 - hence periodic build for the master branch is broken.

Dependencies: None

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Merge deadline: None

Testing instructions:

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Author notes and concerns:

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Reviewers

• TBD