Set GIT_STRATEGY to none by default for DAST

What does this MR do?

This MR resolves issue #10429 (closed)

Since the project's repository is not required to run DAST, set GIT_STRATEGY to none by default to prevent it from being cloned. This can be overridden by setting GIT_STRATEGY to fetch or clone in the projects gitlab-ci.yml

This MR also updates the DAST documentation to explain the change to GIT_STRATEGY

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

• 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