Remove Julia v1.9 get_extension compatibility code

Since all SciML packages now require Julia v1.10+, we can remove the
compatibility checks for `isdefined(Base, :get_extension)` as package
extensions are available in all supported Julia versions.

This removes unnecessary version checks in extension loading code.

Author: ChrisRackauckas

src/OptimizationBase.jl

@@ -4,10 +4,6 @@ using DocStringExtensions
 using Reexport
 @reexport using SciMLBase, ADTypes
 
-if !isdefined(Base, :get_extension)
-    using Requires
-end
-
 using ArrayInterface, Base.Iterators, SparseArrays, LinearAlgebra
 import SciMLBase: OptimizationProblem,
                   OptimizationFunction, ObjSense,