Drop the license fallback and allow_failure from cng-secrets-manager

What does this MR do and why?

The shared QA_EE_LICENSE CI variable has been rotated to a license that carries the secrets_manager add-on (see https://gitlab.com/gitlab-org/gitlab/-/work_items/628661#note_3835761559). The full e2e suites passed against it in !255365, so the temporary scaffolding on the cng-secrets-manager job is no longer needed.

This MR:

• Removes allow_failure: true from cng-secrets-manager in .gitlab/ci/test-on-cng/main.gitlab-ci.yml, so the job blocks again when the Secrets Manager specs fail.
• Removes the before_script override that exported QA_EE_LICENSE="${QA_EE_LICENSE_SECRETS_MANAGER:-$QA_EE_LICENSE}". The QA_EE_LICENSE_SECRETS_MANAGER project variable has been deleted, so the job now inherits the standard .cng-test before_script.
• Updates the error message in qa/qa/ee/support/helpers/secrets_management/secrets_manager_helper.rb so it points at QA_EE_LICENSE instead of the removed variable.

References

• Issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/628661
• Test MR that validated the rotated license: !255365
• MR that introduced the fallback: !255043 (merged)

How to set up and validate locally

Not applicable, this is a CI configuration change. Confirm the cng-secrets-manager job in the e2e:test-on-cng child pipeline of this MR passes without allow_failure.

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