Use PPL by default for Fractional Chromatic Index

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See #23798.

CC:  @dcoudert

Component: **graph theory**

Author: **David Coudert**

Branch/Commit: **[`910fb83`](https://github.com/sagemath/sagetrac-mirror/commit/910fb839eb4612f42bf61858d0f0725fb1f2559c)**

Reviewer: **Jeroen Demeyer**

_Issue created by migration from https://trac.sagemath.org/ticket/24099_

