clean up and fix for dotnet-8

Issue #, if available:

https://learn.microsoft.com/en-us/dotnet/core/compatibility/sdk/8.0/dotnet-pack-config Starting for .NET SDK 8.0, the default configuration for dotnet build is Debug, while default configuration for dotnet pack is Release :)

Description of changes:

set configuration for build and release explicitly, for test, using Debug
Checked the release pipeline, we already set the configuration explicitly to Release, so that it will not affect our pipeline
Removed the unused codebuild scripts as we moved away from codebuild CI.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

awslabs / aws-crt-dotnet