Upgrade reference to AWS SDK for .NET to the recently GA V4 version.

[normj]

Description of changes:
The AWS SDK for .NET recently GA the V4 version. https://aws.amazon.com/blogs/developer/general-availability-of-aws-sdk-for-net-v4-0/

Updating this package to V4 unblocks users like this one aws/aws-sdk-net#3362 (comment) to move their application to V4.

The V4 is an evolutionary major version and we are expecting users to update fairly quickly and relatively short cycle of dual support with V3.

I made the version bump a major version bump since it forces users to update to V4 of the SDK.

Testing
I ran the tests in the solution and scanned the SDK usage for possible null collection issues. In V4 collection properties default to null. Only area I saw that could be a problem was in the GetVersion method of SecretCacheItem which already had a null check so was safe.

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.52%. Comparing base (befeea6) to head (4ae8437).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #169   +/-   ##
=======================================
  Coverage   72.52%   72.52%           
=======================================
  Files           6        6           
  Lines         222      222           
  Branches       23       23           
=======================================
  Hits          161      161           
  Misses         52       52           
  Partials        9        9           

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[simonmarty]

Released as 2.0.0. Thanks for the contribution!