Remove ForwardDiff 1 compat (again)

[pjaap]

The test environment cannot be resolved with this version, see #195 ...
Hence, it is NEVER tested and we should not include this version in the compats.

[pjaap]

I manually ran Pkg.test(; force_latest_compatible_version=true) with this change and it passes.

[j-fu]

The relevant upstream PR is
JuliaDiff/SparseDiffTools.jl#313

It is not clear if this will be fixed as people move on to do sparsity detection via DifferentiationInterface.jl. (discussed on the Julia slack). I think it is time then to replace that dependency...

[gdalle]

It had been prophesized, and now the time has come ;)
#109

[j-fu]

Well, but: JuliaDiff/DifferentiationInterface.jl#774
Though I think I can tolerate the regression (which does not affect core functionality), there seems to be someting that SparseDiffTools is doing better (or I miss some options to tune things).

[gdalle]

Taking a look!

[j-fu]

#203 solves this with respect to direct dependencies as this gets rid of SparseDiffTools. Test dependencies still need to evolve.

[j-fu]

We can close this as the remaining issues were outside of the package and are resolved by now.