Add feature flags endpoints to OpenApi v2 docs

What does this MR do and why?

This merge request adds endpoints from API::FeatureFlags to the OpenApi V2 documentation as per #211512.

Please note that:

• Changes to doc/api/openapi/openapi_v2.yaml were excluded to avoid conflicts within the same documentation effort.
• Errors related to consumes property were ignored as per https://gitlab.com/gitlab-org/gitlab/-/issues/377941.
• Descriptions of all endpoints were updated to match the markdown documentation.
• We use multiple types for params that accept them (even if it's not supported in OpenApi V2) as per !101875 (comment 1148561131).

More guidelines on the changes here can be found in &8926 (comment 1151742900).

Resolves #376905.

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