Fix intermittent 404 errors loading GitLab Pages

What does this MR do and why?

This backports !136970 (merged) and !137062 (merged) to 16-6-stable-ee. This fixes intermittent 404 errors (gitlab-com/gl-infra/production#17120 (closed)) that were caused by !135993 (merged).

This merge request adds two minor migrations to ensure that GitLab Pages deployments are only marked ready when the upload has completed.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).