Docs: Remove arbitrary 1 week wait note for zero-downtime upgrades

What does this MR do?

The note in "Upgrading without downtime" from 3 years ago via gitlab-foss#35939 (closed) carried an arbitrary 1 week wait as a safe period to allow for most background migrations (introduced at the time) to complete. The measure was based on the deployment sizes of GitLab.com back then.

Since that time, we've added sections and commands to check the progress of background migrations directly and users do not need to wait 1 week between each upgrade when deciding to upgrade through more versions quicker (for example, deciding to go from 13.4 to 13.12 in zero downtime style is about 8 weeks of wait if this is strictly followed): omnibus-gitlab#4908 (closed)

This change removes the note about recommending 1 week in favor of using the direct checks, so upgrades can move quicker when necessary.

Related issues

Came up in a Slack thread on #questions: https://gitlab.slack.com/archives/C0AR2KW4B/p1626905194048100

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Ensure that the product tier badge is added to doc's h1.
• Request a review based on the documentation page's metadata and associated Technical Writer.

To avoid having this MR be added to code verification QA issues, don't add these labels: feature, frontend, backend, ~"bug", or database

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.