Run Secret Push Protection spec on self managed environments

What does this MR do and why?

Run Secret Push Protection spec on non-saas environments. As per discussion at https://gitlab.com/gitlab-org/gitlab/-/issues/489863#note_2119390492 , Secret Push protection was enabled for self-managed on 17.2 (see history dropdown at https://docs.gitlab.com/ee/user/application_security/secret_detection/secret_push_protection/ ).

This MR allows the test to run on pre-Staging master pipelines such that any potential failures are caught earlier.

References

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MR acceptance checklist

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How to set up and validate locally

Run against eg. GDK:

QA_DEBUG=true WEBDRIVER_HEADLESS=false bundle exec bin/qa Test::Instance::All http://gdk.test:3000/ ./qa/specs/features/ee/browser_ui/13_secure/secret_push_protection_spec.rb

Run against Staging (where it was currently running):

GITLAB_QA_USER_AGENT =<USER_AGENT> GITLAB_ADMIN_USERNAME=<ADMIN_USERNAME> GITLAB_ADMIN_PASSWORD="<ADMIN_PASSWORD>" GITLAB_USERNAME=<USERNAME> GITLAB_QA_ACCESS_TOKEN="<ACCESS_TOKEN>" GITLAB_PASSWORD="<GITLAB_PASSWORD>"  QA_DEBUG=true WEBDRIVER_HEADLESS=true bundle exec bin/qa Test::Instance::All https://staging.gitlab.com qa/specs/features/ee/browser_ui/13_secure/secret_push_protection_spec.rb