Does any version of Unity currently support .NET 4.7.2 or 4.8?
I am currently on macOS and I have the most up to date Mono and .NET Core and neither are allowing me to use features from 4.7.2 and up. I can see that in my csproj files the Target Framework is set to 4.7.1 and can not be changed since Unity regenerates the csproj files.
Quick Edit: Using Unity 2019.2.13f1
Unity bundles its own Mono and .Net Core versions, the versions installed on your system are only relevant for external tools like editors.
Unity 2020.1a still sets the framework to v4.7.1, check the Experimental Scripting Previews forum and the release notes for upcoming changes. A thread in that forum mentions that C# 8 support is planned for the 2020 release cycle at the earliest.