gh-37555: use Parent in Symbolic Ring
replacing the auld class `CommutativeRing` by `Parent` in the symbolic
ring
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/37555
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
