Transfer ownership of Release Post cover image to the Brand team

The release post manager chooses the cover image for each release post. This task should be revised such that the cover image for each post is a GitLab-branded X.Y image owned by the brand team.

Implementation

• Brand team to populate www-gitlab-com/source/images/release_post_covers/ with X_Y-cover.jpg images @amittner
• ProdOps to update release post template to point to that directory @brhea
• Remove the tasks in the release post process that ask the RPM to choose the image @brhea
• Brand team to implement process to keep folder updated in advance of future releases @amittner