Add Delivery Team as owner of releases posts

Why is this change being made?

In preparation of #7755 that is being activated on 2020-06-25, I've added the Delivery team as CODEOWNERS of:

• /source/releases/posts/ along with Product Managers
• /data/release_managers.yml

Delivery team needs to prepare and deploy blog posts for regular and security patch releases.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

For help with failing pipelines reach out in #mr-buddies in Slack