gh-36574: rename the backtrack algorithm of method `longest_path` with deprecation
Fixes #36530.
More precisely, we:
- Rename algorithm "backtrack" as "heuristic" and add a deprecation
warning
- Raise an error when some parameters are used in combination with the
"backtrack"/"heuristic" algorithm
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URL: https://github.com/sagemath/sage/pull/36574
Reported by: David Coudert
Reviewer(s): David Coudert, Matthias Köppe
