test: raise minimum solver stack for DowngradeCI

[ChrisRackauckas-Claude]

The minimum CPU downgrade environment had two independent resolver issues. Raise SciMLBase to 3.40 in both package and test projects, raise OrdinaryDiffEqSDIRK to 2.8.2, and resolve the root and test projects together. The test graph also needs OrderedCollections 1.x because StateSelection 1.11 requires that compatibility line.

Failing-before evidence:

• Main-branch Downgrade failed resolving OrdinaryDiffEqNonlinearSolve: https://github.com/SciML/DiffEqGPU.jl/actions/runs/32028536447/job/95383076935
• The first PR run reached Pkg.test but could not merge separately generated root/test environments: https://github.com/SciML/DiffEqGPU.jl/actions/runs/32035792118/job/95405802223
• The current rerun reaches the same Pkg.test boundary because the reusable @v1 workflow does not yet contain the secondary-manifest cleanup from Restore project metadata before downgrade QA .github#127: https://github.com/SciML/DiffEqGPU.jl/actions/runs/32038198023/job/95412674461

Verification:

• Combined root/test downgrade resolution completed successfully with OrderedCollections 1.8.1 and StateSelection 1.11.0.
• Local Julia 1.10 CPU test run passed: DAE 15/15 with 1 intentional broken; non-stiff regression 48/48; non-stiff callbacks 78/78; stiff regression 75/75; stiff callbacks 46/46; FiniteDiff 5/5; Auto-Conversion 6/6; public interfaces 19/19; plus the mass-matrix group 2/2.
• git diff --check, actionlint, and typos passed.

No public API changes and no deprecation path is applicable. This draft depends on SciML/.github#127 being merged/released before the Downgrade check can pass. Please ignore this PR until it has been reviewed by @ChrisRackauckas.