Mark AspNetCore projects that aren't packaged explicitly (#18767)

* 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