drm-backend and clients/simple-dmabuf-feedback: Drop support libgbm older than 21.1.1
Since simple-dmabuf-feedback and drm-backend require libgbm 21.1.1, there is no need to support the old libgbm which does not support modifiers and fd import.
Since simple-dmabuf-feedback and drm-backend require libgbm 21.1.1, there is no need to support the old libgbm which does not support modifiers and fd import.