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

ext/DiffEqNoiseProcessReverseDiffExt.jl

@@ -1,7 +1,7 @@
 module DiffEqNoiseProcessReverseDiffExt
 
 using DiffEqNoiseProcess, DiffEqBase, Random
-isdefined(Base, :get_extension) ? (import ReverseDiff) : (import ..ReverseDiff)
+import ReverseDiff
 
 @inline function DiffEqNoiseProcess.wiener_randn(rng::Random.AbstractRNG,
         proto::ReverseDiff.TrackedArray)