Certain methods related to TSP (Travelling salesman problem) fail on immutable graphs:

sage: Graph('EJ~w', immutable=True).longest_cycle().order()
NotImplementedError: an immutable graph does not change name

sage: graphs.ChvatalGraph().copy(immutable=True).is_hamiltonian()
NotImplementedError: an immutable graph does not change name

This PR fixes this by setting the _name attribute directly instead of setting it via name().

Another fix would be to allow changing the name of immutable graphs by removing these two lines from name():

if self.is_immutable():
    raise NotImplementedError("an immutable graph does not change name")

This check was introduced in #15681 and has bitten us in the past. While it seems counterintuitive for immutable graphs to have mutable attributes, the _embedding attribute can already be mutated via set_embedding().

📝 Checklist

• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.