allow fragments to end in either .yml or .yaml

[jmrodri]

Description of the change:
Allow changelog fragments to end in either .yaml or .yml

Motivation for the change:
Some fragments were being added with .yml extensions which caused make test-sanity to report the following warnings:

+ go run ./hack/generate/changelog/gen-changelog.go -validate-only
WARN[0000] Skipping non-YAML file "ansible-metrics-addr.yml" 
WARN[0000] Skipping non-YAML file "rm-ansible-metrics.yml" 

Checklist

If the pull request includes user-facing changes, extra documentation is required:

• Add a new changelog fragment in changelog/fragments (see changelog/fragments/00-template.yaml)
• Add or update relevant sections of the docs website in website/content/en/docs

[jmrodri]

With the changes in this PR, the warnings are now gone:

$ make test-sanity
...
+ go run ./hack/generate/changelog/gen-changelog.go -validate-only
+ git diff --exit-code

[joelanford]

/lgtm