Truncated_GCM_mesh currently not supported

[ilopezgp]

Even though we still have the user-facing option to use a truncated GCM mesh:

truncated_gcm_mesh = TC.parse_namelist(namelist, "grid", "stretch", "flag"; default = false)

we are currently enforcing a standard uniform grid,

z₀, z₁ = FT(0), FT(nz * Δz)
z_stretch = CC.Meshes.Uniform()
return (; z_stretch, z_max = z₁, z_elem = nz)

We should revert/fix this behavior to the version we have in the theta-var branch,

https://github.com/CliMA/TurbulenceConvection.jl/blob/theta-var/driver/main.jl#L269