fix: Generated CDK projects referencing version of System.Text.Json

[normj]

Issue #, if available: https://github.com/aws/aws-toolkit-visual-studio/issues/462

Description of changes: The generated CDK projects have TreatWarningsAsErrors as true which makes deployments start failing immediately when a dependency has a listed vulnerability. In this case System.Text.Json has a listed failure which causes the generated CDK project fail to build preventing deployment.

This PR updates the version of System.Text.Json which was actually coming in as a transitive dependency from Microsoft.Extensions.Configuration.Json. It also removes the TreatWarningsAsErrors property from the generated projects because we have no control when a package might be listed as vulnerable and we don't one deployments to just start failing when that happens.

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 62.35%. Comparing base (b390e37) to head (e2ed984). Report is 2 commits behind head on dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #850 +/- ## ======================================= Coverage 62.35% 62.35% ======================================= Files 279 279 Lines 10900 10900 Branches 1513 1513 ======================================= Hits 6797 6797 Misses 3566 3566 Partials 537 537 ```

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