Use new one-shot AES functionality in .NET 6

[GSPP]

The performance of AES is a critical component to the overall performance of this library. Mega uses a bespoke encryption system which requires many individual AES calls. Optimizations have been performed already but it was not possible to eliminate significant overheads.

.NET 6 promises to provide fast and easy to use AES one-shot APIs: dotnet/runtime#2406

I'm opening this as a tracking issue for doing potential optimization work once .NET 6 is released.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.