Make Julia 1.10 minimum

[fonsp]

Yay!

We should merge and release #3049 first, so that people with Julia 1.6 can get one last Pluto update that includes this warning message.

We also need to fix #2626 (comment)

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="remove-1.6-code")
julia> using Pluto