test_multistack upgrade of CDK to v2

[mbordash2]

Summary

Upgrading the CDK multistack test suite to use the latest version of CDK, which includes an upgrade from V1 to V2.

Why This Is Needed

It is generally best practice to keep software versions up to date and given the potential vulnerability with CDK outlined in the attached issue, this fix will address that as well.

What Changed

Changed

CDK multistack test was updated to latest V2 version of CDK.
CDK bootstrap was upgraded in the test account as well.

Checklist

• Have you followed the guidelines in our Contribution Requirements?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Does your submission pass tests?
• Have you linted your code locally prior to submission?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?
• Have you updated documentation, as applicable?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.20%. Comparing base (0d02431) to head (ee4ec68).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2647      +/-   ##
==========================================
- Coverage   90.21%   90.20%   -0.01%     
==========================================
  Files         256      256              
  Lines       15691    15691              
==========================================
- Hits        14155    14154       -1     
- Misses       1536     1537       +1     

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