sage.rings.finite_rings: Change impl to implementation

[mkoeppe]

Classes in sage.rings.finite_rings use the parameter impl, whereas other Sage classes use implementation.

We propose to change impl to implementation, with deprecation.

This will be helpful for #15223.

CC: @tscrim @kwankyu @roed314 @fchapoton

Component: finite rings

Issue created by migration from https://trac.sagemath.org/ticket/30507

[mkoeppe]

comment:2

Setting new milestone based on a cursory review of ticket status, priority, and last modification date.