Use SECURE_ANALYZERS_PREFIX in DAST API image URL

What does this MR do?

Use the SECURE_ANALYZERS_PREFIX variable in the DAST API image URL. This allows offline users a quick way to change all Secure analyzer image URLS.

• Use SECURE_ANALYZERS_PREFIX
• Remove -engine suffix

Related to #329128 (closed)

CI Templates

• Change is backwards compatible
• No test updates as template logic hasn't changed
• No documentation updated needed
• Test project:
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  • CI Template
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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
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• 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

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• [-] 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