Fix build issue and add support for dotnet 8

[sacOO7]

• Add support for dotnet8 as per LTS announcement
  https://devblogs.microsoft.com/dotnet/announcing-dotnet-8/
• Fix ably agent header issue where agent header is showing as dotnet-standard for dotnet6, 7 and 8.
• Build is failing because of old version of NewtonSoft -> Improper Handling of Exceptional Conditions in Newtonsoft.Json #1170

┆Issue is synchronized with this Jira Task by Unito

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3984

[sacOO7]

• Fixing newtonsoft json will also fix need for having a separate newtonsoft DLL's in Unity package as per unity README : )

[sacOO7]

https://docs.unity3d.com/Packages/com.unity.nuget.newtonsoft-json@3.2/manual/index.html

[sacOO7]

Fixed as a part of #1313, #1309 and #1300