DAST Site validation - Text File Upload - Documentation

Problem to solve

This should update documentation for DAST site validation (using Text File Upload) which is being added by #233020 (closed).

Proposal

• The documentation in https://docs.gitlab.com/ee/user/application_security/dast/#domain-validation will need to be updated to reflect this new process of domain validation.
• A deprecation notice should go out that we are removing the old, insecure way of validating domains and site ownership.

Local Testing

• README
• YouTube Video

Who can address the issue

Other links/references

Main Issue - #233020 (closed)

/cc @rdickenson