.Net: Obsolete SKContext[...] indexer

[shawncal]

Motivation and Context

The SKContext[...] indexer is simply shorthand for SKContext.Variables[...]. With additional routes to doing the same thing, there's more confusion about which to use when. In this case, there's a great deal of confusion about the mutability of these variables and whether they can/should be changed within functions or between functions.

Description

This PR starts to address the above issues by simplifying the SDK surface. The first step is to remove the "extra" route to accessing the same data, the indexer on SKContext.

This is a breaking change. The mitigation is simply to update all uses of context["MY_VALUE"] to context.Variables["MY_VALUE"]

In the PRs that follow, the mutability issue will be addressed by making it clear which variables are input, how and where to set output, and which can/cannot be transformed by an SKFunction.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄
  => This is a breaking change.