gh-35978: Fix GradedModularFormElement multiplication
Fix #35932 - incorrect multiplication for GradedModularFormElement.
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
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- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/35978
Reported by: grhkm21
Reviewer(s): David Ayotte, grhkm21, Matthias Köppe
