[Insights] Using SDK 20.1-preview with some fixes. Fixing reported issues with the Diagnostic Settings cmdlets

[gucalder]

Description

• Using DSK 0.20.1-preview to fix some issues with the Swagger specs (check this file for details about the changes https://github.com/Azure/azure-sdk-for-net/blob/psSdkJson6/src/SDKs/Monitor/changelog.md)
• All cmdlets for Diagnostic Settings were modified to remain backwards compatible, but to not fail if the legacy diagnostic setting (called service) is not present. Now it is possible to list all the diagnostic settings for a resource, it is possible to create a new diagnostic setting even if there is not "service" diagnostic setting defined yet. The remove cmdlet removes the diagnostic settings, regardless of name, if there is only one.

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate change log has been updated
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • the changes have gone through a cmdlet design review
  • the cmdlet markdown files were generated using the platyPS module

[markcowl]

@gucalder Closing in favor of #7021