Since you did not break the fasts, but rather skipped them, you only need to perform qaza for them, kaffarah (fasting for 60 consecutive days) is not mandatory.
Therefore, you must observe 21 qaza fasts to compensate for the missed fasts, and it is advisable to do so as soon as possible. Additionally, seek repentance and forgiveness from Allah (SW).
۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔۔
دلائل:
الھدایة: (240/1، ط: مکتبہ رحمانیہ)
وقضاء رمضان ان شاء فرقہ و ان شاء تابعہ، لاطلاق النص لکن المستحب المتابعۃ مسارعۃ الی اسقاط الواجب بقدر ماادرک۔
و فیھا ایضاً: (242/1، ط: مکتبہ رحمانیہ)
و من اصبح غیر ناویا للصوم فاکل لا کفارۃ علیہ۔
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.