Fix pre-existing CPU downgrade resolver conflict

Summary

The downgrade test lane fails on clean master independently of https://github.com/SciML/DiffEqGPU.jl/pull/489.

Reproduction

At clean master commit b4825a09265d09cac0c8b76e5c6413e4ee0fc9cc, run the repository's Julia 1.10 downgrade test command:

import Pkg; Pkg.test(; coverage=true, julia_args=["--check-bounds=yes", "--compiled-modules=yes", "--depwarn=yes"], force_latest_compatible_version=false, allow_reresolve=false)

The resolver fails before tests run with:

Unsatisfiable requirements detected for package ForwardDiff
ForwardDiff is pinned to 1.0.1 by the downgrade environment
OrdinaryDiffEqRosenbrock 2 with LinearSolve 5.2.0 requires ForwardDiff >= 1.3.3

The parent commit also fails, through a separate LinearSolve 5.0 / DifferentiationInterface conflict, so this is not introduced by PR #489.

Impact

The CPU downgrade check is red on master and on the compat-only PR. The fix should be made in the downgrade floor/pinning logic or the relevant package compat constraints, separately from the SciMLTesting declaration.

Evidence: https://github.com/SciML/DiffEqGPU.jl/actions/runs/31882516251/job/95006963475

Start with the repository's Julia 1.10 downgrade test command and the downgrade floor or pinning logic mentioned in the issue. Trace the ForwardDiff, OrdinaryDiffEqRosenbrock, LinearSolve, and DifferentiationInterface compatibility constraints, then verify that the command resolves dependencies and reaches the tests on clean master without the reported conflicts.