Replace `int` with `long` as ID type in samples/docs/tests

This project originally used `int` (32-bit) as the ID type in resources. While that's not wrong, I believe we should default to `long` (64-bit) in docs, samples and most tests. With many database transactions, the 2 billion limit can be reached earlier than expected, requiring a costly database migration and possible outage. A 64-bit auto-incrementing number doesn't have this limitation. Users can always choose their own ID type, but for anyone who doesn't care, this sounds like the safer default. We should keep a few tests around, though, that still use `int`, so it remains covered. Models in `JsonApiDotNetCoreTests/IntegrationTests/ReadWrite` and `JsonApiDotNetCoreTests/IntegrationTests/AtomicOperations` should not be updated, as they cover scenarios with varying ID types. The change is basically from, for example: ```c# public sealed class TelevisionBroadcast : Identifiable<int> ``` to: ```c# public sealed class TelevisionBroadcast : Identifiable<long> ``` Likewise, tests that parse the ID should be updated accordingly, from: ```c# int newShopId = int.Parse(responseDocument.Data.SingleValue.Id); ``` to: ```c# long newShopId = long.Parse(responseDocument.Data.SingleValue.Id); ``` As well as usages to unknown IDs in tests, such as from: ```c# int id = Unknown.StringId.Int32; ``` to: ```c# long id = Unknown.StringId.Int64; ```