Mktg: 2022-06-27 Add blog post for upgrading PostgreSQL 12 and 13 servers

Closes #13589 (closed)

Quick summary on the purpose of this blog post and why it is labeled as a priority:

We have found a major bug that affects all GitLab instances that use external PostgreSQL servers.

As part of the related P1/S1 issue (gitlab-org/gitlab#364763 (comment 997417523)) we want to also communicate with as many instance administrators as possible that they have to upgrade their servers as soon as possible.

The blog post will also serve as an additional landing page for people unaware of the problem once the get the errors and search for them. It will provide with an explanation and a way to mitigate it.

Checklist for writer

• Link to issue added, and set to close when this MR is merged
• Due date and marketing milestone (e.g. Mktg: 2021-03-28) added for the desired publish date
• Please suggest a target keyword for your post for SEO: ___
• Please add links to three related blog posts, GitLab issues, documentation or other related content so the reader can learn more at the bottom of the post. (We will take care of the formatting.)
• If time sensitive
  • Added ~"priority" label
  • Mentioned @vsilverthorne to give her a heads up ASAP
• Blog post file formatted correctly, including any accompanying images
• All relevant frontmatter included
• Review app checked for any formatting issues
• Reviewed by fellow team member
• If approval before publishing is required
  • Any required internal or external approval for this blog post has been granted (please leave a comment with details)
• Assign to @vsilverthorne OR the Editorial team member who reviewed your pitch issue for final review (If they are on PTO and your post is time sensitive, please share your MR in #content on Slack to ask for another reviewer.)

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