chore: Remove Xenial references in the repo

[ticachica]

Remove references to 'Xenial' from the repo now that Xenial is going to be EOL.

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Summary

Part of the Xenial Build Image Deprecation Project

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[conventional-commit-lint-gcf[bot]]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot https://conventionalcommits.org/