Remove iteration_count as a default Results object value

[mrmundt]

Fixes #3753

Summary/Motivation:

As part of #3753 and #3754, we decided that iteration_count should be removed as a default piece of information on the Results object (particularly because it's not well defined for certain solvers, e.g., MIP solvers). This instead downgrades iteration count to being an extra_info declaration, if desired, and is specific to each solver.

Changes proposed in this PR:

• Remove iteration_count from defaults Results object
• Adjust each individual solver to move iteration_count into the extra_info ConfigDict
• Report ALL iteration counts from highs rather than whatever subset, with appropriate key-value pairs

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