Accept changlog.yml as a valid changelog

[cavcrosby]

Fixes #4105.

I'd like to mention that when looking over #2832 again, I see that changelog.yaml was preferred over changelog.yml because the latter wasn't documented. While this isn't true anymore, changelog.yml appears to be sparsely mentioned (only reference I found in the offical docs was here). This is to say, I wonder if this should be fixed.

[alisonlhart]

I recall now that the reasoning for excluding .yml originally was out of an abundance of caution. antsibull-changelog generates a changelogs/changelog.yaml file specifically, and this is likely the reason a lot of the docs specifically mention .yaml.
Edit: The Ansible core team has confirmed that both extensions are accepted.