[BUG] Fix exposed legacy_renum_only options in public graph APIs

[rlratzel]

A change (still to be merged at the time of this writing) exposes a legacy_renum_only which was previously an internal flag, but is now exposed in order to create graphs specifically for use with algos that use pylibcugraph Graph objects stored in the cuGraph graph objects.

This is confusing to users not familiar with the implementation details, and could cause problems if users don't understand when they should or shouldn't set it.

For reference: #2394 (comment)

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Behavior based on legacy_renum_only setting vs. SG/MG/algo type

	SG legacy algo	MG legacy algo	SG plc algo	MG plc algo
legacy_renum_only=True	ERROR	ERROR	optimal performance	optimal performance
legacy_renum_only=False	no error	no error	no error but reduced performance (redundant renumber step)	no error but reduced performance (redundant renumber step), and potential hang

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