Rayleigh-Brillouin scattering in binary mixtures of disparate-mass constituents: <math> <mrow> <msub> <mi>SF</mi> <mn>6</mn> </msub> <mtext>−</mtext> <mi>He</mi> <mo>,</mo> <mspace width='"0.16em"'></mspace> <msub> <mi>SF</mi> <mn>6</mn> </msub> <mtext>&</mtext></mrow></math>