Validate that changelog bodies are plain strings

[GrahamGoudeau]

https://docs.solo.io/gloo/latest//changelog/open_source/#v1-0-0-rc2

changelog formatting can interfere with each other. only 1-4 are actually numbered, number 5 is a totally different breaking change. We may need changelog bodies to just be plain strings without any kind of fancy markdown

[rickducott]

Not a big fan of plain strings, because I think it's useful to use this and this in changelog entries.