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/RecursiveArrayToolsMeasurementsExt.jl

@@ -1,7 +1,7 @@
 module RecursiveArrayToolsMeasurementsExt
 
 import RecursiveArrayTools
-isdefined(Base, :get_extension) ? (import Measurements) : (import ..Measurements)
+import Measurements
 
 function RecursiveArrayTools.recursive_unitless_bottom_eltype(a::Type{
         <:Measurements.Measurement,

ext/RecursiveArrayToolsMonteCarloMeasurementsExt.jl

@@ -1,8 +1,7 @@
 module RecursiveArrayToolsMonteCarloMeasurementsExt
 
 import RecursiveArrayTools
-isdefined(Base, :get_extension) ? (import MonteCarloMeasurements) :
-(import ..MonteCarloMeasurements)
+import MonteCarloMeasurements
 
 function RecursiveArrayTools.recursive_unitless_bottom_eltype(a::Type{
         <:MonteCarloMeasurements.Particles,

ext/RecursiveArrayToolsTrackerExt.jl

@@ -1,7 +1,7 @@
 module RecursiveArrayToolsTrackerExt
 
 import RecursiveArrayTools
-isdefined(Base, :get_extension) ? (import Tracker) : (import ..Tracker)
+import Tracker
 
 function RecursiveArrayTools.recursivecopy!(b::AbstractArray{T, N},
         a::AbstractArray{T2, N}) where {

ext/RecursiveArrayToolsZygoteExt.jl

@@ -2,7 +2,6 @@ module RecursiveArrayToolsZygoteExt
 
 using RecursiveArrayTools
 
-if isdefined(Base, :get_extension)
     using Zygote
     using Zygote: FillArrays, ChainRulesCore, literal_getproperty, @adjoint
 else