Adding Changelog for Release 4.2.0 (#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

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Co-authored-by: Luc Berger <[email protected]>
Co-authored-by: Carl Pearson <[email protected]>
Co-authored-by: brian-kelley <[email protected]>