Drop support of Julia 1.6

[j-fu]

I plan to drop support of Julia 1.6 for VoronoiFVM.jl and all related packages (e.g. ExtendableGrids etc.). The minimal Julia version to be supported will be 1.9 with its Extension mechanism.

Many other packages did this step, e.g. meanwhile, LinearSolve.jl (a dependency of VoronoiFVM) requires 1.10 as minimal version, which means that Pkg will resolve to older versions of LinearSolve.jl when a lower version is needed.

Probably, Julia 1.10 will become the new LTS (longtime support) release and replace 1.6 in this role.

In general, it is advisable to work with the current stable releases of Julia.