Fix deprecated endpoint OpenAPI handling

Summary

Deprecated REST endpoints have their summary fields prefixed with [DEPRECATED] rather than using the deprecated true grape desc directive.

Implementation Plan

• For each endpoints that manually defines [DEPRECATED], remove that tag.
• Add deprecated true as defined by the grape documentation.
• Update the gitlab-grape-openapi gem to add deprecated: true to the operation in question as defined in the OAPI 3.0.0 specification.