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/PreallocationToolsReverseDiffExt.jl

@@ -1,7 +1,7 @@
 module PreallocationToolsReverseDiffExt
 
 using PreallocationTools
-isdefined(Base, :get_extension) ? (import ReverseDiff) : (import ..ReverseDiff)
+import ReverseDiff
 
 # PreallocationTools https://github.com/SciML/PreallocationTools.jl/issues/39
 function Base.getindex(b::PreallocationTools.LazyBufferCache,