Add info about Compiler Plugin to Migration Guide

[AndreiKingsley]

Compiler Plugin was changed a lot, this all should be reflected in Migration Guide.
Also improve info about outdated plugins: https://kotlin.github.io/dataframe/migration-from-plugins.html

[Jolanrensen]

I would adjust this somewhat. "Was changed a lot", well, yeah, it was created ;P
It has been experimental for a while and changed quite often, so rather than telling people how to migrate from an older version to a newer version of the compiler plugin, I'd focus on migration from gradle/ksp/without plugin -> with compiler plugin.