Remove outdated Julia version checks

Since Julia v1.10 is now LTS, remove version checks for v1.8 and v1.9.
- Remove VERSION >= v"1.9" check in test/downstream/remake_autodiff.jl
- Remove VERSION >= v"1.8" check in test/runtests.jl

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: claude

test/downstream/remake_autodiff.jl

@@ -30,23 +30,20 @@ end
 du01, dp1 = Zygote.gradient(sum_of_solution, u0, p)
 
 # These tests depend on a ZygoteRule in a package extension
-# package exentsions do not exist before 1.9, so they cannot work.
-if VERSION >= v"1.9"
-    function symbolic_indexing(u0, p)
-        _prob = remake(prob, u0 = u0, p = p)
-        soln = solve(_prob, Tsit5(), reltol = 1e-6, abstol = 1e-6, saveat = 0.1,
-            sensealg = BacksolveAdjoint(autojacvec = ZygoteVJP()))
-        sum(soln[x])
-    end
-
-    du01, dp1 = Zygote.gradient(symbolic_indexing, u0, p)
-
-    function symbolic_indexing_observed(u0, p)
-        _prob = remake(prob, u0 = u0, p = p)
-        soln = solve(_prob, Tsit5(), reltol = 1e-6, abstol = 1e-6, saveat = 0.1,
-            sensealg = BacksolveAdjoint(autojacvec = ZygoteVJP()))
-        sum(soln[o, i] for i in 1:length(soln))
-    end
-
-    du01, dp1 = Zygote.gradient(symbolic_indexing_observed, u0, p)
+function symbolic_indexing(u0, p)
+    _prob = remake(prob, u0 = u0, p = p)
+    soln = solve(_prob, Tsit5(), reltol = 1e-6, abstol = 1e-6, saveat = 0.1,
+        sensealg = BacksolveAdjoint(autojacvec = ZygoteVJP()))
+    sum(soln[x])
+end
+
+du01, dp1 = Zygote.gradient(symbolic_indexing, u0, p)
+
+function symbolic_indexing_observed(u0, p)
+    _prob = remake(prob, u0 = u0, p = p)
+    soln = solve(_prob, Tsit5(), reltol = 1e-6, abstol = 1e-6, saveat = 0.1,
+        sensealg = BacksolveAdjoint(autojacvec = ZygoteVJP()))
+    sum(soln[o, i] for i in 1:length(soln))
 end
+
+du01, dp1 = Zygote.gradient(symbolic_indexing_observed, u0, p)

test/runtests.jl

@@ -89,10 +89,8 @@ end
         @time @safetestset "Ensemble with DifferentialEquations automatic algorithm selection" begin
             include("downstream/ensemble_diffeq.jl")
         end
-        if VERSION >= v"1.8"
-            @time @safetestset "Solution Indexing" begin
-                include("downstream/solution_interface.jl")
-            end
+        @time @safetestset "Solution Indexing" begin
+            include("downstream/solution_interface.jl")
         end
         @time @safetestset "Unitful interpolations" begin
             include("downstream/unitful_interpolations.jl")