A Class of Luminescent Cyclometalated Alkynylgold(III) Complexes: Synthesis, Characterization, and Electrochemical, Photophysical, and Computational Studies of [Au(C∧N∧C)(C⋮CR)] (C∧N∧C = κ3C,N,C Bis-cyclometalated 2,6-Diphenylpyridyl)