Clean up deprecated Activity Log solution

# Rule request

## Suggested rule change

Activity Log solution within Log Analytics is deprecated. This solution can be cleaned up.

## Applies to the following

The rule applies to the following:

- Resource type: **Microsoft.OperationsManagement/solutions**
- Product: _OMSGallery/AzureActivity_

## Additional context

[Deprecation of Activity Log solution](https://docs.microsoft.com/en-us/azure/azure-monitor/platform/activity-log-collect#activity-logs-analytics-monitoring-solution)
[Azure template reference](https://docs.microsoft.com/en-us/azure/templates/microsoft.operationsmanagement/2015-11-01-preview/solutions)

Start with the linked Azure deprecation notice and template reference, then locate the rule covering Microsoft.OperationsManagement/solutions with product OMSGallery/AzureActivity. Confirm how the existing rule is represented and update its coverage so the deprecated solution is cleaned up, using any nearby validation guidance you find to verify the result.

Written by the indexing model from the issue text.