Measurement of the $CP$-violating phase $φ_\mathrm{s}$ in the B$^0_\mathrm{s}$ $\to$ J$/ψ\, φ$(1020) $\to$ $μ^+μ^-$K$^+$K$^-$ channel in proton-proton collisions at $\sqrt{s} =$ 13 TeV