Change remaining SPKG.txt to SPKG.rst, delete remaining SPKG changelogs #30955
Description
Follow-up from #29633 and #29636.
CC: @jhpalmieri @seblabbe @Etn40ff
Component: documentation
Keywords: sd111
Author: Matthias Koeppe
Branch/Commit: 224059a
Reviewer: Sébastien Labbé, Frédéric Chapoton
Issue created by migration from https://trac.sagemath.org/ticket/30955