fix: downgrade API version

[oZakari]

Overview/Summary

This pull request includes a change to the infra-as-code/bicep/modules/policy/assignments/policyAssignmentManagementGroup.bicep file. The change updates the version of the Microsoft.Authorization/policyAssignments resource.

Version update:

• infra-as-code/bicep/modules/policy/assignments/policyAssignmentManagementGroup.bicep: Updated the resource version from 2025-01-01 to 2024-05-01.

Closes #949

Breaking Changes

None

Testing Evidence

Replace this with any testing evidence to show that your Pull Request works/fixes as described and planned (include screenshots, if appropriate).

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