Rename DAST API to API Security Testing throughout the docs

Problem to solve

The "DAST API" feature was renamed "API security testing". For details, see gitlab-com/www-gitlab-com!133238 (merged).

Proposal

The aim of this MR is to apply this renaming to the GitLab documentation.

To split this into manageable chunks, the work is divided into:

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".
   • Rename all instances to API security testing (!150239 - merged)
2. Move all "API security testing" files to a directory named api_security_testing.
   • Redirect from DAST API to API security testing (!151306 - merged)
3. Update the docs site's left navigation pane to replace all instances of "DAST API" with "API security testing".
   • Rename docs nav entries to API security testing (gitlab-docs!4696 - merged)
4. Check for links in the release notes to "DAST API" and replace them with the new directory.

Who can address the issue

Anyone.

Other links/references

N/A