Remove dependency on Newtonsoft.Json from Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost project

[tmat]

Use System.Text.Json instead.

[jasonmalinowski]

@tmat This is a dupe of #71020, but I won't close this right now in case that breaks the active PR.

[tmat]

Ah, cool. I just created this one so that Copilot can work on it. Can mark #71020 dup of this ;)