Add changelog for multilibs solver support

[ci skip]

changelog.d/public-libs-solver

+synopsis: Add support for multiple public libraries to the solver
+packages: cabal-install
+issues: #6039
+prs: #6836 #6812 #6047
+description: {
+cabal-install can now take into account the presence and visibility of
+sublibraries when solving. This also means that, when using cabal-install,
+multiple public libraries support is extended to GHCs older than 8.8.
+The support is limited to source packages for now.
+}
+