Move defaults into script for DAST API

What does this MR do?

Move variable defaults into script section of DAST API job. This makes it easier to customize the job by end users, and removed the requirement that DAST API configuration variables must be set globally.

Some variables that are used by the job have been removed and are no longer available for use:

• DAST_API_REPORT
• DAST_API_REPORT_ASSET_PATH

Related to #324734 (closed)

CI Template

• Changes are not backwards compatible. Changes made to latest template version.
• No documentation updates are needed (docs still going through review)
• No test changes are needed as the template logic has not changed
• Test project:
  • Project
  • CI template
  • Job
  • 

Screenshots (strongly suggested)

No screenshot added as latest templates do not display in the UX.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team