stack.yaml: bump to lts-6.14
stack.yaml: bump to lts-6.14

MichielDerhaeg/stack,mrkkrp/stack,martin-kolinek/stack,Fuuzetsu/stack,anton-dessiatov/stack,mrkkrp/stack,anton-dessiatov/stack,deech/stack,AndreasPK/stack,MichielDerhaeg/stack,deech/stack,juhp/stack,Fuuzetsu/stack,juhp/stack,martin-kolinek/stack,AndreasPK/stack