There are 4 airports in Corsica. Air Corsica is the local airline.
In the North, Haute Corse:
Calvi and Bastia To the north, the closest airport to Calenzana is Calvi. With Air France or Air Corsica, you can reach Paris in 1.5 hours or Marseille and Nice in 1 hour. By landing in Calvi, you will be 13km from Calenzana, the northern starting point of the GR20.
South:
Ajaccio and Figari In southern Corsica, you will be able to land or depart from Ajaccio or Figari (between Porto-Vecchio and Bonifacio).
Figari airport is the closest to Conca (40km south). You will have no trouble finding a plane to Paris, Marseille or Nice When it comes to your plane ticket, it may be wise not to take the round trip at the same airport.
If you take the GR20 in the North South direction, you could for example land in Calvi and take the plane back to Figari.