Redirect from DAST API to API security testing

What does this MR do?

• Move content of the dast_api directory to the api_security_testing directory.
• Add redirects for all Markdown (.md) files previously in the dast_api directory to the matching files in the api_security_testing directory.

The scope of this MR is limited to step 2 of the parent issue. The remaining steps will be addressed in separate MRs.

1. Find and replace all instances of "DAST API" in the docs with "API security testing". Check for and remove any duplicate words, such as "testing testing" or "test testing".
2. THIS MR -> Move all "API security testing" files to a directory named api_security_testing.
3. Update the docs site's left navigation pane to replace all instances of "DAST API" with "API security testing".
4. Check for links in the release notes to "DAST API" and replace them with the new directory.

Related issues

Rename DAST API to API Security Testing through... (#457449)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product tier badge under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's 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 you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• 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.
  • Ensure a release milestone is set.
  • 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 reviews above. Maintainer's review can occur before or after a technical writer review.