build: replace OrdinaryDiffEq with OrdinaryDiffEqCore

[oameye]

resolves #596

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[oameye]

I guess the IntegrationTests should be dealt with downstream. The CI / test (Interface2, pre) test seem to fail due to a seed being different on CI then locally or something. Seems unrelated to the PR.

[ChrisRackauckas]

yeah that one is slightly stochastic.

[oameye]

Shall we tag a new release?