Migrate ABDF2 eulercache to ESDIRKIMEXCache, drop legacy IE caches

[singhharsh1708]

Summary

Migrates ABDF2’s first-step ImplicitEuler fallback to the unified ESDIRKIMEXCache path and removes the legacy ImplicitEulerCache / ImplicitEulerConstantCache from OrdinaryDiffEqSDIRK.

This was the last remaining non-generic SDIRK cache path.

Verification

• ABDF2 matches master bit-exactly (naccept, u_end) for both IIP/OOP
• SDIRK convergence: 82 pass / 2 broken
• DAE suite: 8/8 pass
• Existing FBDF/DFBDF allocation failures are unchanged and pre-existing

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.

[ChrisRackauckas]

Needs a new minimum on the Project.toml.

[singhharsh1708]

@ChrisRackauckas done..