Drop unsupported DiffEqCallbacks 2 and 3 compat

[ChrisRackauckas-Claude]

Summary

• require DiffEqCallbacks 4 in ModelingToolkitBase
• remove the stale 2.x and 3.x compatibility lines, which cannot coexist with ModelingToolkitBase's SciMLBase 3.19 floor

Validation

The registry constraints cap DiffEqCallbacks 2.x and 3.x at SciMLBase 2.x. An explicit DiffEqCallbacks 3.9.1 plus SciMLBase 3.19 resolver probe is unsatisfiable for that reason.

I generated the deployed alldeps minimum graph on the unmodified base, then repeated it after this one-line compat edit. Both graphs contain 298 packages, have an empty package/version/tree diff, and select DiffEqCallbacks 4.16.0 with SciMLBase 3.19.0. The two removed majors therefore do not represent a valid or exercised minimum graph.

I then ran the real ModelingToolkitBase InterfaceI group on Julia 1.10.11 with the candidate manifest locked (allow_reresolve=false):

Test Summary: | Pass  Broken  Total      Time
InterfaceI    | 1469       5   1474  22m09.0s
     Testing ModelingToolkitBase tests passed

Additional checks:

• candidate manifest SHA-256 remained 852da7673a38b322c171b4400ea28d1faf47d02e8d4c7f390808df1343373a25 during the locked test
• repository-wide Runic check: passed (exit 0)
• TOML parse: passed
• git diff --check: passed

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

[ChrisRackauckas-Claude]

Full CI classification after completion:

• The intended root Downgrade / Downgrade Tests - InterfaceI job passes, as do Runic and runic-suggestions.
• Both Catalyst Modeling jobs reproduce the existing parent_sys failure tracked by Catalyst#1516.
• The ModelingToolkitStandardLibrary downstream job reproduces the existing AccelerationSensor structural-singularity failure tracked by ModelingToolkitStandardLibrary#489.
• ModelingToolkitBase downgrade/InterfaceII failures reproduce the existing BoundaryValueDiffEq MIRK failure tracked by Add NullParaemeters default. #542.
• ModelingToolkitBase QA reproduces the existing Aqua ambiguity/piracy failures tracked by QA: track pre-existing Aqua/JET/ExplicitImports findings (root + ModelingToolkitBase) #4670.
• The root QA runner was lost while its test step was still marked in_progress; GitHub exposes no log artifact (404), with all cleanup steps left pending.

Thus the DiffEqCallbacks compat target is green and no remaining failure is introduced by this one-line change.