sagemathgh-39269: add parameter immutable to generators in `sage/graphs/digraph_generators.py` (part 3)
    
Following sagemath#39264 and sagemath#39266, we add parameter immutable to the remaining
digraph generators.

This PR is motivated by discussions in sagemath#39177.

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### ⌛ Dependencies

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URL: sagemath#39269
Reported by: David Coudert
Reviewer(s): David Coudert, Frédéric Chapoton