Always run frontend_fixture as-if-foss when MR has frontend and feature flag labels

What does this MR do and why?

This MR creates rspec-all frontend_fixture as-if-foss job in MR pipelines when frontend and feature flag labels are applied.

This will prevent a master FOSS broken issue like #382998 (closed) which happened when a feature flag was moved to ee/, but it was still referenced in CE codebase.

With this change, when a feature flag is moved but is still referenced in CE codebase, rspec-all frontend_fixture as-if-foss job fails with InvalidFeatureFlagError.

Closes #382998 (closed)

Testing

Tested changes in !104320 (closed).

Code changes	Pipeline	Is InvalidFeatureFlagError detected?
Feature flag entry change with frontend feature flag (870913f3)	✅ rspec-all frontend_fixture as-if-foss is created: pipelines/702534041	✅ Yes. rspec-all frontend_fixture as-if-foss fails with Feature::InvalidFeatureFlagError: Missing feature definition for pipeline_name.
Regression: Feature flag entry change with only feature flag (870913f3)	✅ rspec-all frontend_fixture as-if-foss is NOT created: pipelines/702553413	N/A

How to validate

1. Create an MR and move a feature flag yaml file used in CE codebase to ee/config/feature_flags/* directory, e.g. !104320 (closed).
2. Set a target branch to 382998-always-run-frontend-fixture-as-if-foss-when-feature-flag-is-changed for the MR.
3. Ensure frontend and feature flag are applied to the MR.
4. Run a new pipeline.

MR acceptance checklist

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