gh-35245: GenericGraph.adjacency_matrix: using sort=True when getting vertices
### 📚 Description
As illustrated by the following code, the method `adjacency_matrix` of
the GenericGraph class failed when vertices where not sortable.
```python
G = Graph()
G.add_vertices ([14, 'test'])
G.adjacency_matrix()
```
I fixed the problem by using `sort=False` instead of `sort=True` in the
line that get the list of vertices.
I did not open an issue for this bug, but it is similar to #35168 (fixed
by PR #35170)
**NOTE**
I started to write a test to cover the changes but all others test are
broken by this small commit. Indeed the adjacency matrix obtained with
the new code can have a different row/column order. I am not sure what
is the correct way to handle this situation
### 📝 Checklist
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### ⌛ Dependencies
URL: https://github.com/sagemath/sage/pull/35245
Reported by: cyrilbouvier
Reviewer(s): cyrilbouvier, David Coudert
