.NET 8 Support

• Added support for .NET 8
• Removed .NET 6 star wars api, added .NET 8 version

Bug Fixes

• Fixed an issue with SchemaItemPath causing a race condition when attempting to build multiple test server instances simultaniously (@Fgruntjes)
• Fixed a bug with records not being allowed to used as OBJECT graph types in some scenarios
• Fixed a bug with using the [GraphType] attribute on structs. You should now be able to supply a properly customized the name for a graph type built from a struct.
• Fixed a bug where by a [BatchTypeExtension] or [TypeExtension] may fail to build introspection data properly when referencing a graph type with a customized name.
• Fixed a bug where field executions would not always obey schema isolation rules in some scenarios.

Other Changes

• Made the message parameter on GraphController.NotFound(message) optional. A default, generic message will be included in the query errors when not supplied.

Related PRs: #132, #135, #137, #138, #139, #140