Deprecate DAST domain validation variable
What does this MR do?
Adds a deprecation notice to the DAST_FULL_SCAN_DOMAIN_VALIDATION_REQUIRED variable in the DAST docs. The work to deprecate the domain validation feature is being done in %13.11, but the feature was deprecated in %13.8. See gitlab-com/www-gitlab-com!71418 (merged) for the deprecation release post
The main domain validation docs section already has a deprecation notice, but this variable was missed