Replace custom implementation of CopyStream with CopyTo

[danielmarbach]

Description

Replaces the custom implementation of CopyStream with the stream based CopyTo method that has more efficient implementations.

Motivation and Context

Testing

I ran benchmarks against MemoryStream since that seems to be the most used version of Stream in the cases I quickly checked against

Screenshots (if appropriate)

https://github.com/danielmarbach/aws-sdk-net-benchmarks/blob/main/StreamCopy.cs

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license

[normj]

Approved now we are not target .NET Framework 3.5 and CopyTo always exist in all of V4's targets.