Simplify changelog

[nflaig]

changelog format is annoying, we need to simplify that, client specific columns are unused, also would be nice if we can have a way for this to not conflict between branches, using https://github.com/OffchainLabs/unclog could help there

[james-prysm]

our team at prysm uses this, we require adding a changelog file with each pr that contains a header ( from that is pre defined) and in markdown, then adding a bulleted list based on the header such as added, fixed, changed, ignored, etc. there is a function from unclog to put it together into one big change log when releasing and then you update the summary and it auto deletes all the changelog files that were merged.