Downgrade: raise RecursiveArrayTools compat floor to 4.2.0 (InterfaceI downgrade resolve)

[ChrisRackauckas-Claude]

Problem

The Downgrade workflow (Downgrade Tests - InterfaceI) fails at resolution with:

Unsatisfiable requirements detected for package RecursiveArrayTools [731186ca]:
 ├─restricted to versions 4.2.0 - 4 by OrdinaryDiffEqTsit5, leaving only versions: 4.2.0 - 4.3.1
 └─restricted to versions 4.0.0 by an explicit requirement — no versions left

RecursiveArrayTools is declared in the root OrdinaryDiffEq Project.toml only as a
test dependency ([extras] / [targets].test) with no [compat] entry. As a result
julia-downgrade-compat pins it to the bare major floor 4.0.0.

Every in-tree sublibrary (OrdinaryDiffEqTsit5, OrdinaryDiffEqRosenbrock, etc.) is
pulled in via [sources] path-deps and is therefore skipped from downgrade — it stays
at its current version, which requires RecursiveArrayTools >= 4.2.0. So the downgrade
pin of 4.0.0 is below what the sublibraries require, and the resolve has no solution.

The conflict is downgrade-specific: the latest-versions CI does not fail in InterfaceI
for this reason (the resolver picks a modern RecursiveArrayTools there).

Fix

Add an explicit [compat] RecursiveArrayTools = "4.2.0" to the root Project.toml. 4.2.0
is the smallest RecursiveArrayTools co-installable with the sublibraries (which all pin
>= 4.2.0), so it is a genuine minimum, not an arbitrary bump. No upper bound is touched.

Verification (resolution only, not full test suite)

Ran the actual julia-actions/julia-downgrade-compat (Resolver.jl --min=@deps) on
Julia 1.10 (the LTS downgrade floor), with the same effective skip list the SciML
centralized workflow uses (stdlibs ∪ all [sources] sublib names):

• Before (no RecursiveArrayTools compat): the merged InterfaceI resolve selects
  RecursiveArrayTools 4.0.0, which is incompatible with the in-tree
  OrdinaryDiffEqTsit5 (>= 4.2.0).
• After (RecursiveArrayTools = "4.2.0"): the merged resolve selects
  RecursiveArrayTools 4.2.0, co-installable with the sublibraries.

CI will confirm the full build/test.

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This PR should be ignored until reviewed by @ChrisRackauckas.

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