ci: Rename main workflow and use .yaml extension

We no longer have two separate workflows, so rename `full_ci` to just
`ci`.

Additionally, `.yaml` is the preferred extension [1], so rename the
other `.yml` file to `.yaml`.

[1]: https://yaml.org/faq.html

(backport <#4124>)
(cherry picked from commit 4c96512)

Author: tgross35