Doc: The Secret-Detection.gitlab-ci.yml include requires test stage

What does this MR do?

This MR updates the docs regarding including Security/Secret-Detection.gitlab-ci.yml because there is a hidden dependency. The include file assumes a stage of test (which may be fine because it is one of the default stages), but I had a project that did not have a test stage, and the job from this include file failed. The failure message states that "the chosen stage" was not found in the CI configuration, but it does not say which stage could not be found.

This documentation update clarifies the requirement.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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