Disable SAST on project creation in QA spec during the experiment

What does this MR do and why?

The MR updates qa/specs/features/browser_ui/1_manage/project/create_project_spec.rb to disable SAST check on project creation when it exists. new_project_sast_enabled experiment is planned to be enabled on Staging and Production. In this case we can't check for FF state because this is not a feature flag but an experiment and while it's running there will be 3 states: control (default project creation page), candidate and free_indicator (new Enable Static Application Security Testing (SAST) checkbox similar to initialize with readme which is enabled by default) (see https://gitlab.com/gitlab-org/gitlab/-/issues/333196 for further details and screenshot).

Because of the above, proposing to disable SAST checkbox when it exists to make sure we don't block the deployment when this experiment is enabled (as it happened before #342744 (comment 699268308)). I've considered to change expectation text based on existence of the new SAST checkbox but decided not to go with this route as this functionality is only considered to be added and once it's added the spec can be properly updated.

• Related to this QA failure #342744 (closed)

Screenshots or screen recordings

Example of checkbox when `free_indicator` variant is enabled

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Details how to enable experiment in GDK for testing #342744 (comment 706928870)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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