Raise downgrade compatibility floors

[ChrisRackauckas-Claude]

Please ignore this PR until it has been reviewed by @ChrisRackauckas.

Summary

Raise the minimum compatible versions needed by DiffEqGPU's CPU downgrade lane:

• CommonSolve 0.2.6
• DiffEqBase 7.2
• RecursiveArrayTools 4.2
• SciMLBase 3.18
• test-only ModelingToolkit 11.30.1

Root cause and impact

The previous floors admitted an internally incompatible minimum stack. In particular, the older IntegratorBase/OrdinaryDiffEqCore combination failed during downgrade precompilation. These are conservative lower-bound corrections; supported major-version lines remain unchanged.

Local validation

• Ran the exact Julia 1.10 downgrade resolver and workflow-equivalent build/test command with GROUP=CPU.
• Observed exit code 0 and DiffEqGPU tests passed, including 48 non-stiff regression, 78 non-stiff callback, 75 stiff regression, 46 stiff callback, 15 DAE mass-matrix, and the remaining CPU testsets.
• Ran julia -m Runic --check --diff --verbose .: all 70 Julia files passed.
• Parsed both modified Project.toml files with Julia's TOML stdlib and ran git diff --check.

Process

Reproduced the current downgrade failure at the default-branch SHA, adjusted only the floors required by the minimum resolver/test stack, reverted exploratory larger bounds, and reran the complete CPU downgrade suite.