Fix returncodes and stats for iterative solvers

[termi-official]

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

Trying to resolve #507 . However, this is technically a breaking change.

Also, the factorization solvers should set their retcode to Success and not to Default, or am I missing something? Because SciMLBase.successful_retcode(ReturnCode.Default) == false .

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 24.57%. Comparing base (175a74f) to head (2e44544).

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #508       +/-   ##
===========================================
- Coverage   62.61%   24.57%   -38.05%     
===========================================
  Files          29       29               
  Lines        2247     2238        -9     
===========================================
- Hits         1407      550      -857     
- Misses        840     1688      +848     

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[ChrisRackauckas]

Test failure is just some tolerance thing in adjoints, will merge.