* Mark AspNetCore projects that aren't packaged explicitly
- avoid NU5104 warnings due to confusing versioning
- `$(IsShippingPackage)` was semantically incorrect in any case
* Remove redundant `$(IsShippingPackage)` settings in `$(IsAspNetCoreApp)` projects
- default is `true` for all implementation projects
* Use `$(IsPackable)` when deciding how `$(IsAspNetCoreApp)` projects are handled
- remove all use of `$(IsShippingPackage)` for shared framework composition
- update documentation to match these changes
nits:
- remove odd default for `$(IsPackable)` in Directory.Build.targets
- no longer relevant since all `$(IsAspNetCoreApp)` projects are `$(IsShippingPackage)` too
- include more information in docs/ProjectProperties.md
* Add direct System.Text.Json references
- avoid MSB3277 warnings
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