Remove future support for DAST_API_OPENAPI

What does this MR do?

this merge request ensures the DAST_API_OPENAPI environment variable will not be nsupported in the next major version.

What are the relevant issue numbers?

DAST and API Security both use DAST_API_OPENAPI environment variable

GitLab Docs MR

Document DAST_API_SPECIFICATION must be used

src/config/configuration_parser.py

@@ -70,6 +70,9 @@ class ConfigurationParser:
             auth_auto_environment_variables = ['DAST_AUTH_AUTO']
             request_headers_environment_variables = ['DAST_REQUEST_HEADERS']
 
+        if dast_major_version >= 4:
+            api_spec_environment_variables = [dast_api_specification]
+
         description = """
 Run GitLab DAST against a target website of your choice.