Fixed medium triangle unicode for fold-stars

Symbols ⯈ ,⯆ have now been replaced by their equivalent symbols ⏵ ⏷ in the function org-modern-fold-stars The previous are not rendered even on the GitHub page <img width="538" alt="image" src="https://github.com/user-attachments/assets/9b4d6874-843b-4be9-820d-66c7396a7a53" /> After fix, <img width="538" alt="image" src="https://github.com/user-attachments/assets/2adc3d6f-6529-4bd5-a784-360b4d22a0bd" />