Document DAST_API_SPECIFICATION must be used

What does this MR do?

this merge request deletes reference to DAST_API_OPENAPI and deletes a note about DAST_API_SPECIFICATION being removed. this is necessary because DAST_API_OPENAPI results conflicting behaviour in dast-api.

whilst on the surface this looks like a breaking change, the dast ci template has never supported DAST_API_OPENAPI. meaning it has been in a broken state for some time. the change here will fix the issue for more customers than any it will cause.

Related issues

DAST and API Security both use DAST_API_OPENAPI environment variable

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