Add 'security' tag to SAST and Dependency-Scanning Templates

What does this MR do?

Adds the 'security' tag to SAST and Dependency-Scanning CI templates. We use tagged runners and extending the SAST templates with a tag is causing the job to execute twice in the pipeline. This seemed like an easier fix than asking for reverse tag injection.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

This should not need testing. Validated that SAST template behavior is correct with built-in GitLab CI linter.

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

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