Downgrade: raise ModelingToolkit (9.7.1) and OrdinaryDiffEq (6.80.0) compat floors

[ChrisRackauckas-Claude]

Problem

The Downgrade job (LTS Julia 1.10, strict allow_reresolve: false) pins every direct dependency to its declared [compat] lower bound, then builds and tests. The test suite loads MethodOfLines (a [extras] test dependency with no [compat] entry).

At the old floors, the downgrade resolve was Unsatisfiable:

ERROR: LoadError: Unsatisfiable requirements detected for package MethodOfLines [94925ecb]:
 MethodOfLines log:
 ├─restricted by compatibility requirements with SymbolicUtils to versions: [0.8.3-0.8.4, 0.9.0-0.11.0]
 └─restricted by compatibility requirements with ModelingToolkit to versions: uninstalled — no versions left
   └─ModelingToolkit restricted to versions 9.0.0 by an explicit requirement

The conflicting pins (why the old floors were impossible)

1. ModelingToolkit = "9, 10, 11" floors MTK at 9.0.0. But no published MethodOfLines version supports MTK 9.0.0 — the lowest, MethodOfLines 0.11.0, requires ModelingToolkit = "9.7.1 - 9". So at MTK 9.0.0, MethodOfLines is uninstallable. (Older MethodOfLines only support MTK 8.)
2. Raising the MTK floor to 9.7.1 lets MethodOfLines 0.11.0 install, but MTK 9.7.1 requires OrdinaryDiffEq = "6.73.0 - 6", while the old OrdinaryDiffEq = "6, 7" floor pins 6.0.0 — too low.
3. OrdinaryDiffEq 6.73.0 itself still fails: it requires FastBroadcast = "0.2", whereas OrdinaryDiffEqSDIRK (floor 1.2.0) requires FastBroadcast = "0.3.5 - 0.3" — no overlap. OrdinaryDiffEq 6.80.0 is the first version that admits FastBroadcast = "0.2 - 0.3", which overlaps SDIRK's 0.3.5. So 6.80.0 is the smallest co-installable OrdinaryDiffEq floor.

OrdinaryDiffEqSDIRK (1.2.0) and SciMLBase (2.69.0) floors are left unchanged — they co-install fine once MTK = 9.7.1 and OrdinaryDiffEq = 6.80.0.

Fix

Raise two [compat] lower bounds to the smallest versions that resolve at the downgrade minimum:

Dep	Old floor	New floor	Reason
ModelingToolkit	9	9.7.1	lowest MTK any MethodOfLines (0.11.0) supports
OrdinaryDiffEq	6	6.80.0	MTK 9.7.1 needs ODE ≥ 6.73.0; 6.80.0 is the first that shares a FastBroadcast with the SDIRK floor

This raises only lower bounds; no upper bound is touched, no test logic or tolerance is changed.

Resolve verification

Reproduced locally on Julia 1.10 (the LTS downgrade floor):

• Old floors (ModelingToolkit = 9, OrdinaryDiffEq = 6): pinning every direct dep to its floor → Unsatisfiable requirements detected for package MethodOfLines (matches CI).
• New floors: pinning every direct dep to its floor, and separately running julia-downgrade-compat's downgrade.jl in the real deps/LTS configuration, both resolve successfully — manifest pins MTK 9.7.1, MethodOfLines 0.11.0, OrdinaryDiffEq 6.80.0, OrdinaryDiffEqSDIRK 1.2.0, SciMLBase 2.69.0.

Master's non-downgrade CI is unaffected (the package already resolves at the top of its compat ranges; the master CI red was only a Codecov upload-auth error, not a test failure).

Ignore until reviewed by @ChrisRackauckas.