Change Enforce-GR-KeyVault policy assignment from platform management to platform scope

[alperkar]

Overview/Summary

According to https://github.com/Azure/Enterprise-Scale/wiki/ALZ-Policies#platform Enforce-GR-KeyVault policy assignment should be scoped to Platform management group. However, it was being assigned to the management management group. This fixes that.

This PR fixes/adds/changes/removes

Changes Enforce-GR-KeyVault policy assignment from Management to Platform management group

Breaking Changes

none

Testing Evidence

not tested

As part of this Pull Request I have

• Read the Contribution Guide and ensured this PR is compliant with the guide
• Ensured the resource API versions in .bicep file/s I am adding/editing are using the latest API version possible
• Checked for duplicate Pull Requests
• Associated it with relevant GitHub Issues
• (ALZ Bicep Core Team Only) Associated it with relevant ADO Items
• Ensured my code/branch is up-to-date with the latest changes in the main branch
• Performed testing and provided evidence.
• Updated one or more of the following tests (if required)
  • Unit
  • Linting
  • E2E (End-To-End)
  • ValidateAzCloud (Base validation in Azure Cloud)
  • ValidateMcCloud (Base validation in Azure China Cloud)
• Updated relevant and associated documentation (e.g. Contribution Guide, Module READMEs, Wiki Docs etc.)
• If relevant, created or updated Code Tours here

[alperkar]

Fixes #751

[alperkar]

@microsoft-github-policy-service agree

[oZakari]

/azp run validateazcloud

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[oZakari]

LGTM, thanks much @achechen!