[3.7] Bump Azure Pipelines to ubuntu-20.04

[hugovk]

The 3.7 branch builds on Azure Pipelines is failing, for example:

##[warning]An image label with the label ubuntu-16.04 does not exist. ,##[error]The remote provider was unable to process the request. 

• https://dev.azure.com/Python/cpython/_build/results?buildId=118919&view=logs&j=341b3d8e-0d13-57d4-859b-65d31dcbc29e
• [3.7] Update copyright years to 2023. (gh-100848). #100853

The main and 3.11-3.8 branches were already bumped to ubuntu-20.04 by #89170, which didn't make it back to 3.7.

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Alternatively, we could use ubuntu-22.04 for all branches, or the ubuntu-latest alias which is now pointing to ubuntu-22.04.

• https://github.com/actions/runner-images#available-images

[hugovk]

Also included #21776 to move the triggers into the YAML files, which was backported down to 3.8, but not 3.7.

@zooba Do you know why this PR isn't triggering on Azure Pipelines?

[zooba]

The PR trigger is overridden to only run on PRs to main, to avoid slowing down backports. And there's a separate build definition for CI builds (using the same YAML, and the default triggers for CI). I'm pretty sure triggers are only evaluated from the default branch's YAML, which is probablymain.

Updating the OS images seems fine. I'd prefer to have it explicitly select an Ubuntu version - using latest is unfortunately asking for random warnings that may cause the build to look like a failure,though it is handy to get early notification of failures on the next OS... no way to win, unfortunately, but I'd rather not get the "about to change -latest" warnings here I think.

[hugovk]

Please see PR #101089 to replace this one.