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/LinearSolveIterativeSolversExt.jl

@@ -4,7 +4,6 @@ using LinearSolve, LinearAlgebra
 using LinearSolve: LinearCache, DEFAULT_PRECS
 import LinearSolve: IterativeSolversJL
 
-if isdefined(Base, :get_extension)
     using IterativeSolvers
 else
     using ..IterativeSolvers

test/runtests.jl

@@ -4,7 +4,7 @@ const LONGER_TESTS = false
 
 const GROUP = get(ENV, "GROUP", "All")
 
-const HAS_EXTENSIONS = isdefined(Base, :get_extension)
+const HAS_EXTENSIONS = true
 
 if GROUP == "All" || GROUP == "Core"
     @time @safetestset "Basic Tests" include("basictests.jl")