V14: Migrate Newtonsoft to System.Text in value converters

[nikolajlauridsen]

Migrates the value converters to use System.Text.Json instead of Newtonsoft.Json

Additionally, this removes the old JsonNetSerializer since it was a newtonsoft implementation.

This broke some tests because they contained invalid JSON, this is worth being aware of, for the most part there's not much we can do about it, but we may want to consider turning AllowTrailingCommas on in the settings for the SystemTetxJsonSerializer since this may prevent some future issues.

Also removes Nested Content and Grid value converter and so on.

Some classes still have the DataMember attributes, this is because Newtonsoft is still used directly some places, this will be remedied in later PRs

Breaking changes

This PR removes the Newtonsoft implementations of the IJsonSerializer and the contextually aware implementations, this means that Newtonsoft will no longer be used, instead System.Text will, a complete table of differences can be found here.