Adding Changelog for Release 4.2.0 (kokkos#2031)

* Adding Changelog for Release 4.2.0

Part of Kokkos C++ Performance Portability Programming EcoSystem 4.2

* Formatting the changelog a bit more

Mentioning more clearly LAPACK vs BLAS, grouping PRs by logical work unit, etc...

* Remove minor revisions, improve text descriptions

* Changelog: add spmv perftest detail

---------

Co-authored-by: Luc Berger <lberge@sandia.gov>
Co-authored-by: Carl Pearson <cwpearson@users.noreply.github.com>
Co-authored-by: brian-kelley <brian.honda11@gmail.com>