Update Rfc2898DeriveBytes documentation for salt length minimums NET5-/NET6+

https://github.com/dotnet/runtime/pull/57006 is about to remove the minimum length checks for the salt array.

https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Security.Cryptography/Rfc2898DeriveBytes.xml#L129-L152 (and the other overloads) mention this minimum in both the exception descriptions and the remarks.

I'm guessing the exception text would be something like
```diff
- The specified salt size is smaller than 8 bytes or the iteration count is less than 1.
+ The iteration count is less than 1, or (.NET 5 and older) the specified salt size is smaller than 8 bytes.
```

But that seems a little awkward, so maybe the right answer is a more complex split.

Since it doesn't seem obvious to me, I decided to create the tracking issue rather than just attempt something.