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.
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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.