Detect unused feature flags during tests execution

What does this MR do?

Gather all used feature flags by touch'ing file in a tmp/feature_flags/.

Print as part of rspec:coverage that a given feature flag is unused.

This does a very simple workflow:

• when rspec uses a feature flag we touch a file in tmp/feature_flags/<flag-name>.used
• as part of rspec:coverage we look at all feature flags and find ones that are unused
• we fail the rspec:coverage and ask to remove this feature flag

Resolves #232777 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content