Add deprecation of HAVE_UMFPACK to changelog
This was an oversight in the MR !1294 (merged) that came up recently in #379 (closed): The preprocessor constant HAVE_UMFPACK
is deprecated and will be removed until the next release 2.11. Use the macro HAVE_SUITESPARSE_UMFPACK
instead. This change is already merged. This MR adds the corresponding changelog entry.