blog: Add post about scaling Git object database maintenance

In 15.11, Gitaly has introduced a new housekeeping strategy to maintain the object database. This new strategy promises to significantly reduce overhead when maintaining large monorepositories. Add a blog post that describes this new feature that was made generally available in GitLab 16.0.

Closes #34076 (closed).

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 to title for the desired publish date
• If time sensitive
  • Added ~"priority" label if blog needs to run in the next few days
  • Mentioned @sgittlen to give her a heads up ASAP
• Blog post file formatted correctly, including uploading and embedding in markdown any accompanying images
• All relevant frontmatter included
• Review app checked for any formatting issues
• Reviewed by fellow team member(s)
• Reviewer(s) marked approved before sending to blog team
  • Any required internal or external approval for this blog post has been granted (please leave a comment with details)
  • Reviews by Legal team if necessary according to SAFE guidelines
• Assign to @Sgittlen for final review

After the blog has been published:

• Share on your social media channels
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