5.0 Release Branch

[SteveBronder]

Description

We want to merge #2995 and #2928 which are both backwards breaking changes

#2995 removes the _log() lpdf functions, which the compiler already errors on when users try to use them
#2928 allows reverse mode matrix functions to return arena_matrix types, which has implications for memory safety we did not previously have

Are there more things we want to merge for a major release? I made a branch here we could merge everything for 5.0 into before we do a final large merge before release. But if it is just these two then I think we should just merge the directly into develop

Current Version:

v4.8.1

[SteveBronder]

I think #3027 also fits in this