Update net45 to net462

[jon-armen]

Description of changes:

• Remove support for net45 (obsolete)
• Add support for net462

Extracted work from #223 to reduce the amount of change in a single PR. Will update #223 for netcoreapp3.1 and net6.0 changes separately.

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

[jon-armen]

@srprash , I've updated this pr to go into the 3.x line if we want to move this work to that branch.

[srprash]

Hi @jon-armen
Yes, we would like to trace the breaking changes in the 3.x branch. Thanks for doing this. :)

[jon-armen]

@srprash , handled the merge issues. Once we start moving these pr's thru, I'll work towards updating to support dotnet6.

[jon-armen]

@srprash - I've fixed the commits squashing this change to a single commit.

[jon-armen]

This obsoletes #101

[jon-armen]

@srprash - I've merged this down to a single commit. Are we in a position where we can merge into 3.x?

[jon-armen]

@srprash , in order to update to the latest aws sdk (#315), we'll need to drop 4.5 support. Can we start a aws-sdk 4.0 branch for several breaking changes?

[Edit] - Just getting my bearings back around here. I see we're still on version 2.x, and 3.x was tracking the breaking changes. Can we continue using 3.x to track breaking changes?