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/DiffEqBaseGTPSAExt.jl

@@ -1,6 +1,5 @@
 module DiffEqBaseGTPSAExt
 
-if isdefined(Base, :get_extension)
     using DiffEqBase
     import DiffEqBase: value, ODE_DEFAULT_NORM
     using GTPSA

ext/DiffEqBaseGeneralizedGeneratedExt.jl

@@ -1,6 +1,5 @@
 module DiffEqBaseGeneralizedGeneratedExt
 
-if isdefined(Base, :get_extension)
     using DiffEqBase
     using GeneralizedGenerated
 else

ext/DiffEqBaseMPIExt.jl

@@ -1,6 +1,5 @@
 module DiffEqBaseMPIExt
 
-if isdefined(Base, :get_extension)
     using DiffEqBase
     import MPI
 else

ext/DiffEqBaseMeasurementsExt.jl

@@ -1,6 +1,5 @@
 module DiffEqBaseMeasurementsExt
 
-if isdefined(Base, :get_extension)
     using DiffEqBase
     import DiffEqBase: value
     using Measurements

ext/DiffEqBaseMonteCarloMeasurementsExt.jl

@@ -1,6 +1,5 @@
 module DiffEqBaseMonteCarloMeasurementsExt
 
-if isdefined(Base, :get_extension)
     using DiffEqBase
     import DiffEqBase: value
     using MonteCarloMeasurements

ext/DiffEqBaseTrackerExt.jl

@@ -1,6 +1,5 @@
 module DiffEqBaseTrackerExt
 
-if isdefined(Base, :get_extension)
     using DiffEqBase
     import DiffEqBase: value
     import Tracker

ext/DiffEqBaseUnitfulExt.jl

@@ -1,6 +1,5 @@
 module DiffEqBaseUnitfulExt
 
-if isdefined(Base, :get_extension)
     using DiffEqBase
     import DiffEqBase: value
     using Unitful