Refactor Deprecations page to use releases.yml
There is now a SSoT YAML file with release dates: https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/master/data/releases.yml
Let's use this rather than maintaining our own release_dates.json file when building the Deprecations page. We can fetch the YAML file as part of the build process and translate to JSON for our frontend code to pick up.
See also: #1648 (comment 1564739092)