Migrate to .NET Core 6

[lahm86]

This covers stage 1 of the migration to .NET Core 6, with the minimum *.cs changes needed to comply.

• Build action now uses publish rather than build and uses the new format for specifying the output folder. The publish profile details are in the csproj file - so this generates a single file now and we no longer need to move the DLLs into a sub-folder.
  See https://learn.microsoft.com/en-us/dotnet/core/compatibility/sdk/7.0/solution-level-output-no-longer-valid
• Replaced WindowsAPICodePack UI dialogs with those built-in (this was used previously as it was the only way to access the more user-friendly directory picker).
• Updated use of obsolete properties/classes in UpdateChecker.cs and App.xaml.cs.
• Fixed issues when launching external processes related to UseShellExecute.
  See https://learn.microsoft.com/en-us/dotnet/core/compatibility/fx-core#change-in-default-value-of-useshellexecute
• External dependencies migrated to .NET Core 6 - RectanglePacker and TRGE.
• Updated Newtonsoft to 13.0.3

Those projects that have net6.0-windows targets use - either directly or indirectly - System.Drawing.Bitmap. We may migrate to a different .NET Core universal drawing solution in the future, but this will be a big task in itself, and currently we only target Windows anyway so it's not a deal-breaker.

The using update in TR2Room is to fix a compiler error; stage 2 of the migration will be to fully update all usings to the minimum required.

[rr-]

Do you think it'd be useful to have similar class for file dialogs?

[lahm86]

Good idea, a bit tidier and useful if we need to change something again in the future. New commit in with the update.