Downgrade: raise SciMLBase compat floor to 2.49 (2.0.0 floor unresolvable with solver stack)

[ChrisRackauckas-Claude]

The MINIMUM-VERSION (downgrade) job fails. The failure surfaces at the Pkg.test sandbox re-resolve, but it is a pure resolution conflict (reproduced locally on Julia 1.10, the LTS downgrade floor):

Unsatisfiable requirements detected for package DiffEqBase [2b5f629d]:
 ...
 │   └─restricted by compatibility requirements with SciMLBase [0bca4576]
 │     to versions: [4.0.0-6.6.6, 6.56.0-6.68.2] or uninstalled, leaving only versions: 6.56.0-6.66.0
 │       └─SciMLBase restricted to versions 2.0.0 by an explicit requirement
 └─restricted by compatibility requirements with SciMLBase [0bca4576]
   to versions: 3.13.2-6.53.6 or uninstalled — no versions left

and, probing minimal floors directly, Unsatisfiable ... OrdinaryDiffEqSDIRK [2d112036] ... restricted by SciMLBase to versions: uninstalled.

Why the old floor was impossible

• The package compat is SciMLBase = "2, 3.1", so julia-downgrade-compat pins SciMLBase to the floor of the first range, 2.0.0.
• The test target pulls the full OrdinaryDiffEq / MethodOfLines / ModelingToolkit stack. Those test deps carry no [compat], so they are not downgrade-pinned and resolve to recent versions, dragging in a recent DiffEqBase + OrdinaryDiffEq sublibraries.
• Modern DiffEqBase declares SciMLBase lower bounds far above 2.0.0 (registry: DiffEqBase 6.130.1-6.132 -> SciMLBase 2.0.1, climbing higher for newer DiffEqBase), and OrdinaryDiffEqSDIRK at the resolved version has no SciMLBase 2.0.x in range. So SciMLBase 2.0.0 has an empty intersection with the co-installed solver stack -> Unsatisfiable.

(Master latest-versions CI is unaffected: the Core groups on julia 1 and julia lts pass; this is downgrade-specific.)

Fix

Raise the lower bound of the first range: SciMLBase = "2, 3.1" -> SciMLBase = "2.49, 3.1". The author intent to support both the 2.x and 3.1 series is preserved; only the unresolvable low end of 2.x is dropped. No upper bound lowered, no test logic touched.

Resolve verification (local, Julia 1.10 LTS), resolution only

Mimicking the downgrade pin (SciMLBase pinned to floor, test stack left to resolve):

SciMLBase floor	result
2.0.0	Unsatisfiable (DiffEqBase / OrdinaryDiffEqSDIRK)
2.0.1	Unsatisfiable
2.30.0	Unsatisfiable
2.48.0	Unsatisfiable
2.49.0	RESOLVE SUCCEEDED
2.50.0	RESOLVE SUCCEEDED
3.1.0	RESOLVE SUCCEEDED

2.49.0 is the smallest co-installable floor with the current solver stack. The full test suite was not run locally; PR CI confirms.

Note: there is a separate, pre-existing QA-group failure on lts (Aqua project_extras: Project.toml vs test/Project.toml mismatch) unrelated to this resolution fix.

Ignore until reviewed by @ChrisRackauckas.