sagemathgh-39247: add parameter immutable to generators in `src/sage/…

…graphs/graph_generators.py`

    
We add parameter `immutable` to all generators in
`src/sage/graphs/graph_generators.py`.

As suggested in sagemath#39177, all methods creating and returning a graph
should offer the option to get an immutable graph.


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### ⌛ Dependencies

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URL: sagemath#39247
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton