gh-36187: use semi-primitive root when checking kernel polynomials of isogenies
The existing code takes a generating set of the unit group, but as a
comment in the code suggests, a set of generators for $(\mathbb
Z/m)^\times/\pm$ suffices. The existing function `_least_semi_primitive`
computes such a semiprimitive root when $m$ is an odd prime power, so we
may use it.
URL: https://github.com/sagemath/sage/pull/36187
Reported by: Lorenz Panny
Reviewer(s): Travis Scrimshaw
