Drop support for Julia 1.10 and 1.11

[svchb]

When the first Version of Julia is released after the next LTS Version of Julia we should drop support for 1.10 and 1.11.
This issue just contains notes for what legacy things to remove:

Legacy version for OrdinaryDiffEqCore < 3 (OrdinaryDiffEq < v6.106.0)
function OrdinaryDiffEqCore.alg_cache(alg::SymplecticPositionVerlet, u, rate_prototype,

[efaulhaber]

Why should we drop support for no reason? A lot of people are still using Julia 1.10 and 1.11. We can think about dropping support when we run into problems that require extra effort to fix on legacy versions.

Legacy version for OrdinaryDiffEqCore < 3 (OrdinaryDiffEq < v6.106.0)

What does that have to do with Julia 1.10 and 1.11? Users can still load an old version of OrdinaryDiffEqCore with a new version of Julia.