Define cts_remake to remake ODEProblem manually

SciMLBase changed the way remake(::AbstractODEProblem,...) works in
2.68.0. Previously, if the `f` kwarg was an AbstractODEFunction,
the problem was remade with f directly as its function. After 2.68.0,
remake(::AbstractODEProblem) calls a new remake method with
`f` and `prob.f` as arguments. This new function returns an
AbstarctSciMLFunction of the same type and properties as `prob.f`.

This commit replicates the behavior of DiffEqBase.remake on the provided
arguments before the change.

src/solvers/multirate.jl

@@ -24,16 +24,32 @@ struct MultirateCache{OC, II}
     innerinteg::II
 end
 
+"""
+    cts_remake(prob::DiffEqBase.AbstractODEProblem; f::DiffEqBase.AbstractODEFunction)
+
+Remake an ODE problem with a new function `f`.
+"""
+function cts_remake(prob::DiffEqBase.AbstractODEProblem; f::DiffEqBase.AbstractODEFunction)
+    return DiffEqBase.ODEProblem{DiffEqBase.isinplace(prob)}(
+        f,
+        prob.u0,
+        prob.tspan,
+        prob.p,
+        prob.problem_type;
+        prob.kwargs...,
+    )
+end
+
 function init_cache(prob::DiffEqBase.AbstractODEProblem, alg::Multirate; dt, fast_dt, kwargs...)
 
     @assert prob.f isa DiffEqBase.SplitFunction
 
     # subproblems
-    outerprob = DiffEqBase.remake(prob; f = prob.f.f2)
+    outerprob = cts_remake(prob; f = prob.f.f1)
     outercache = init_cache(outerprob, alg.slow)
 
     innerfun = init_inner(prob, outercache, dt)
-    innerprob = DiffEqBase.remake(prob; f = innerfun)
+    innerprob = cts_remake(prob; f = innerfun)
     innerinteg = DiffEqBase.init(innerprob, alg.fast; dt = fast_dt, kwargs...)
     return MultirateCache(outercache, innerinteg)
 end