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/AMDGPUExt.jl

@@ -1,5 +1,5 @@
 module AMDGPUExt
-isdefined(Base, :get_extension) ? (using AMDGPU) : (using ..AMDGPU)
+using AMDGPU
 import DiffEqGPU
 
 using .AMDGPU

ext/CUDAExt.jl

@@ -1,5 +1,5 @@
 module CUDAExt
-isdefined(Base, :get_extension) ? (using CUDA) : (using ..CUDA)
+using CUDA
 import DiffEqGPU
 
 using .CUDA

ext/MetalExt.jl

@@ -1,5 +1,5 @@
 module MetalExt
-isdefined(Base, :get_extension) ? (using Metal) : (using ..Metal)
+using Metal
 import DiffEqGPU
 
 using .Metal

ext/oneAPIExt.jl

@@ -1,5 +1,5 @@
 module oneAPIExt
-isdefined(Base, :get_extension) ? (using oneAPI) : (using ..oneAPI)
+using oneAPI
 import DiffEqGPU
 
 using .oneAPI

src/DiffEqGPU.jl

@@ -83,17 +83,4 @@ export GPURosenbrock23, GPURodas4, GPURodas5P, GPUKvaerno3, GPUKvaerno5
 export terminate!
 
 # This symbol is only defined on Julia versions that support extensions
-if !isdefined(Base, :get_extension)
-    using Requires
-end
-
-@static if !isdefined(Base, :get_extension)
-    function __init__()
-        @require CUDA="052768ef-5323-5732-b1bb-66c8b64840ba" include("../ext/CUDAExt.jl")
-        @require AMDGPU="21141c5a-9bdb-4563-92ae-f87d6854732e" include("../ext/AMDGPUExt.jl")
-        @require oneAPI="8f75cd03-7ff8-4ecb-9b8f-daf728133b1b" include("../ext/oneAPIExt.jl")
-        @require Metal="dde4c033-4e86-420c-a63e-0dd931031962" include("../ext/MetalExt.jl")
-    end
-end
-
-end # module
+@static end # module