Remove very old redirect files

What does this MR do?

With the effort to implement the new GitLab Pages redirect system, the initial limit was a 32k _redirects file. When we created the initial file for the docs project, it's over 41k already. I think the redirect files in /update should be removed prior to this change, and not added to the _redirects file. This will reduce the number of entries (and thus the file size) by 25%+, which will bring us back under the 32k limit, which means we may not need the increase the 64k.

We could take a look at the metrics, and perhaps only delete a portion of these, but I think the benefits greatly outweigh any cons. I also believe this will speed up some of our linting tests, as we'll have ~147 fewer links to test, and ~147 fewer files to scan with markdownlint and Vale.

One reason I think we can go ahead with this, is the note at the top of https://docs.gitlab.com/ee/update/upgrading_from_ce_to_ee.html, which guides anyone to the old location (in an old stable branch), if they want to see that. In any case, we certainly don't need to keep redirect docs for non-existent update guides for versions that are 3+ years old and completely unsupported... Everyone should be using the standard update guide anyways, which is what you'll find if you search for updating GitLab docs.

Related issues

Related to #30718 (closed)

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
  • Ensure that the product tier badge is added to doc's h1.
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group labels, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add the product tier badge accordingly.
• Add GitLab's version history note(s).
• Add/update the feature flag section.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
  • Ensure docs metadata are present and up-to-date.
  • Ensure Technical Writing and documentation are added.
  • Add the corresponding docs:: scoped label.
  • Add twdoing when starting work on the MR.
  • Add twfinished if Technical Writing team work on the MR is complete but it remains open.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.