[chore] small tweaks & bump VMs in CI

[kelset]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

(wasn't really sure what to pick tbh)

Summary

While digging around the code I noticed that the ADO vms used by this repo are a bit outdated. So I tweaked a bit the vars.yml file, added all the referenced VMs, and updated to newer versions.
I've matched the "latest" ones but used the fixed reference, according to the docs: https://docs.microsoft.com/en-us/azure/devops/pipelines/agents/hosted?view=azure-devops&tabs=yaml#software
This is aligned with what react-native-test-app is using: https://github.com/microsoft/react-native-test-app/blob/trunk/.github/workflows/build.yml

Moreover, I've set the Xcode version to match the one set as default by the new macos VM: https://github.com/actions/virtual-environments/blob/main/images/macos/macos-11-Readme.md#xcode

This is actually something we might want to discuss about, since technically in the CircleCI config we should also change to match, potentially: https://github.com/microsoft/react-native-macos/blob/main/.circleci/config.yml#L55

I didn't change it since it seems to be a config we have in place just because we're a fork of core and they have a CircleCI config? https://github.com/facebook/react-native/blob/1b31d6bb582768ccbe92d3c1a9e43354a8c531e5/.circleci/config.yml#L55

So wanted to check before diverging from upstream.

To sum it up:

• ubuntu-18.04 -> ubuntu-20.04
• macOS-10.15 -> macOS-11
• Xcode_12_4 -> Xcode_13_1

Test Plan

Running on CI is itself the test, let's see how it goes :)

[kelset]

Update on this PR - I spent some time trying to get to the bottom of the CI ❌

• The android one I think is still happening because the fix by @amgleitman has been merged only against 0.66 branch: Add --autogenerateVersionNumber option to bump-oss-version.js #989
• The iOS ones seem to be actually something to look more into, as I can repro locally:

I did some basic digging around but I couldn't find anything specific that could address it, so I'll go into the 0.66 branch, verify if it happens there, and adapt accordingly. Will report back afterwards.

This is because I noticed that CI seems faulty in main:

• [chore] address codeowners to reflect current status #996
• but not in 0.66 branch: Add --autogenerateVersionNumber option to bump-oss-version.js #989