Delete old redirect files in admin/analytics/api docs

What does this MR do?

Deletes old redirect files that no longer need to be maintained. They are more than 3 months old, and redirects will still happen on docs.gitlab.com.

Covers redirects in:

• /doc/administration
• /doc/analytics
• /doc/api
• /doc/monitoring -> Added in this MR as there were some non-standard redirecting between /doc/administration/monitoring and /doc/monitoring, causing pipeline failures.

I did a git grep to search the complete codebase for links to every file in this MR. There are no links to any of these files in the UI, code comments, or other docs.

I also made added FREE to the vale acronym check, (needed due to our new badges, like **(FREE SELF)**), so that vale runs on all files. Otherwise vale will try to run on the deleted files and fail (see failed pipelines).

Related issues

Related to #297337 (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.
  • If working on UI text, add the corresponding UI Text 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.

For more information about labels, see Technical Writing workflows - Labels.

For suggestions that you are confident don't need to be reviewed, change them locally and push a commit directly to save others from unneeded reviews. For example:

• Clear typos, like this is a typpo.
• Minor issues, like single quotes instead of double quotes, Oxford commas, and periods.

For more information, see our documentation on Merging a merge request.

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.