[3.7] Bump Azure Pipelines to ubuntu-20.04 by hugovk · Pull Request #100854 · python/cpython

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 probably main.

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.