Disallow feature flags to use licensed names

What does this MR do?

Part of: #259611 (closed)

Soft-change we still use implicit check, but we disallow of adding new.

Disallow of feature flags to use licensed names

This ensures that all feature flags are explicitly used, and have a unique meaning.

This work does:

• hide licensed from bin/feature-flag
• discovers the feature flag names to collide with licensed feature names
• disallows to use licensed names in feature flag API (unless force=1)
• removes mentions about implicit feature flags in documentation

This does not:

• remove implicit feature flag check (just in case someone still uses it)
• the removal of implicit feature flag will be done at some point later

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