NetSDKCredentialsFile is marked as obsolete

[jaszb]

StoredProfileAWSCredentials is now marked as obsolete.

We should be using

Amazon.Runtime.CredentialManagement.NetSDKCredentialsFile or Amazon.Runtime.CredentialManagement.SharedCredentialsFile

[adamhathcock]

This is a non-issue as the StoredProfileAWSCredentials class is only used in the sample code. Which credentials method used doesn't matter.