Sort release post items alphabetically by stage

Why is this change being made?

Issue: #7826 (closed)

Problem

We rely on filenames to define the order of release post items. (See: https://gitlab.com/gitlab-com/www-gitlab-com/-/tree/master/data/release_posts/13_0).

Updating files is a very tedious process.

Solution

We predefine the order of post items. In this case, the names of files do not matter anymore. They will be automatically sorted based on the proposal.