CompatHelper: bump compat for SciMLBase to 3, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the SciMLBase package from 2 to 2, 3.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[github-actions]

Turing.jl documentation for PR #2802 is available at:
https://TuringLang.github.io/Turing.jl/previews/PR2802/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.45%. Comparing base (ab29465) to head (d2a7721).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2802   +/-   ##
=======================================
  Coverage   86.45%   86.45%           
=======================================
  Files          22       22           
  Lines        1454     1454           
=======================================
  Hits         1257     1257           
  Misses        197      197           

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[penelopeysm]

Note that the successful CI run was with SciMLBase@2

[penelopeysm]

This PR cannot be merged: Optimization is not yet compatible with SciMLBase@3

Edit: see SciML/Optimization.jl#1179 for upstream progress

[ChrisRackauckas-Claude]

Courtesy note from the SciMLBase v3 ecosystem sweep: this PR is low-risk from the v3 breaking-surface perspective. A grep of src/ and test/ for the main v3 flags came up clean — no u_modified! call sites, no DEProblem type references, no 3-arg ensemble prob_func, no single-int timeseries sol[i] indexing. Turing's SciMLBase usage appears limited to the OptimizationSolution type in src/optimisation/Optimisation.jl, which is unaffected by v3.

One ecosystem-wide caveat: Pkg.resolve() fails on any env containing SciMLBase 3.1 right now because OrdinaryDiffEq still pins RecursiveArrayTools ≤ 3.54 while SciMLBase 3.1 requires RAT v4. Expect CI here to fail at the Pkg.add step until upstream OrdinaryDiffEq releases a RAT-v4-compatible version; re-running afterward should go green.

For reference, the SciML-side pattern PR with full breaking-change context is SciML/DiffEqCallbacks.jl#300.

cc @ChrisRackauckas