Rename DAST API to API security testing

What does this MR do?

Change all instances of "DAST API" in the documentation to "API security testing".

In scope:

• Rename all instances in the docs of "DAST API" to "API security testing".
• Move all API security testing documentation to the new directory api_security_testing.
  • Added redirection files for every markdown file moved to the new directory.
• Raise a separate MR to apply the renaming to the docs site's left navigation bar.

Changes to UI text are technically in scope, but there are no links in the UI to the API Security Testing documentation.

Out of scope:

Anything requiring code changes, except for UI text files, specifically:

• API security testing output (e.g. log output).
• CI/CD variables specific to API security testing.
• References to the API security testing CI/CD template - DAST-API.gitlab-ci.yml

For a before and after comparison of file names, also numbers of files and directories, see !148136 (comment 1851770271).

Related issues

Rename feature: DAST API to API Security Testing (gitlab-com/www-gitlab-com!133238 - merged)

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.