Merge master and dependency updates into develop

[AArnott]

• fixing issue with backing field naming and serialization failing
• Remove commented code
• Verify empty dictionary is fine to deserialize
• Do for loop only if any elements
• [Formatting] Respect to nesting
• Compute formatters only if elements are there
• enable ignoring fields by using [NonSerialized]
• Dependency review to enable optimisations. #1811 tweak dependencies.
• Add conditions for polyfil dependencies so are not promoted from being transitive.
• Drop dependency to v6
• Fix build breaks
• Avoid unsupported net461 target framework
• Add .NET Framework target to drop dependencies