Remove usage of ReadOnlyCollection as property wrapper #6201

It is often wasted effort to try and protect developers from themselves. Instead of using ReadOnlyCollection then just use IEnumerable.

Making NLog more AOT-friendly. Places that needs some attention:

• SimpleLayout.Renderer - Created SimpleLayout - Marked Renderers-property as obsolete #6200 to mark obsolete
• LoggingConfiguration.ConfiguredNamedTargets
• LoggingConfiguration.AllTargets
• LoggingRule.Levels