CompatHelper: bump compat for OrdinaryDiffEqSSPRK to 2 for package test, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the OrdinaryDiffEqSSPRK package from 1.5 to 1.5, 2 for package test.
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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.54%. Comparing base (8549b94) to head (994947b).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #409      +/-   ##
==========================================
- Coverage   93.09%   91.54%   -1.55%     
==========================================
  Files          42       42              
  Lines        6805     6804       -1     
==========================================
- Hits         6335     6229     -106     
- Misses        470      575     +105     

Flag	Coverage Δ
unittests	91.54% <ø> (-1.55%)	⬇️

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[JoshuaLampert]

Failing CI should be unrelated to this PR and be fixed by #410.

[JoshuaLampert]

Can be closed since #411 is merged.