The “State Employes’ Retirement Act”, as it now exists or may hereafter be amended, is hereby adopted for the City of Santa Monica, and plenary authority and power are hereby vested in said City, its City Council and its several officers, agents and employes to do and perform any act, or exercise any authority granted, permitted, or required under the provisions of said Retirement Act, to enable said City to become or continue as a contracting City participating in the State Employes’ Retirement System; provided, however, that the City Council may terminate any contract entered into with the Board of Administration of the State Employes’ Retirement System only under authority granted by ordinance adopted by a majority vote of the electors of the City of Santa Monica, voting on such proposition at an election at which such proposal is presented.