Drop support for Julia < 1.6

[prbzrg]

We did it for CI in #13, but compat should be updated too.

[prbzrg]

Some AD packages already did that.

lower bound is v1.9: https://github.com/SciML/ModelingToolkit.jl/blob/d3a0dc5e7761768b4205c77eb46049983706d30f/Project.toml#L107

lower bound is v1.6: https://github.com/FluxML/Zygote.jl/blob/f24b9b263bef7dd8dd75b5ef684f365f953d639e/Project.toml#L61C14-L61C14

https://github.com/JuliaDiff/ForwardDiff.jl/blob/d3002093beb88ff0b98ed178377961dfd55c1247/Project.toml#L35

https://github.com/JuliaDiff/FiniteDiff.jl/blob/50ebf923b2b98bacdc857aeec27c1d805289906f/Project.toml#L28

https://github.com/EnzymeAD/Enzyme.jl/blob/922dda320c1ea26542e4bc27d8727a1984d2ed18/Project.toml#L27

https://github.com/FluxML/Tracker.jl/blob/1e7fca1e996a55e385e47ec72f24893cc9d11d46/Project.toml#L45

https://github.com/JuliaDiff/SparseDiffTools.jl/blob/9a713c6b665732d5d993651727204b39c97d5a61/Project.toml#L62

lower bound is v1: https://github.com/JuliaDiff/ReverseDiff.jl/blob/b669294586b0789ab10bbda223543bcec64cf8aa/Project.toml#L32

https://github.com/JuliaDiff/FiniteDifferences.jl/blob/99ad77f05bdf6c023b249025dbb8edc746d52b4f/Project.toml#L22