Updated supported target frameworks

[jon-armen]

*Issue #, if available: #222

*Description of changes:

• Target Frameworks

  • Remove support for net45 (obsolete)
  • Remove support for netcoreapp2.0 (obsolete)
  • Add support for net462
  • Add support for net472
  • Add support for net48
  • Add additional support for netcoreapp3.1 (added to additional Handlers)
  • Add support for net6.0

• AWS SDK

  • Upgrade AWS SDK from v3.3 to v3.7

• Related Issues

  • How about updating Least .NET Framework version to .NET452 #97 Update .net framework version to 4.5.2 #101 .Net SDK release v3.0.0 #102 This will update the frameworks to require 4.6.2 at a minimum since 4.5.2 will be out of support come April 2022

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