Web: MSDeploy definition is invalid

[`MSDeploy`](https://github.com/Azure/azure-rest-api-specs/blob/27cc07ddd294d98e05cb301e07a72378df9f87e8/specification/web/resource-manager/Microsoft.Web/stable/2020-06-01/WebApps.json#L22177-L22201) definition in the Web RP has a `properties` property that defines both `$ref` and `properties` at the same time:

```
"properties": {
"$ref": "#/definitions/MSDeployCore",
"description": "Core resource properties",
"properties": {
"addOnPackages": {
"description": "List of Add-On packages. Add-On packages implicitly enable the Do Not Delete MSDeploy rule.",
"type": "array",
"items": {
"$ref": "#/definitions/MSDeployCore"
}
}
},
```

This is invalid. If the intension is to make a union of properties from both sources, `allOf` should be used instead.

Open specification/WebApps.json around lines 22177-22201 and inspect the MSDeploy definition and its MSDeployCore reference. Validate the corrected schema with the repository's OpenAPI tooling, and confirm that the definition is valid while retaining both the referenced and additional properties.

Written by the indexing model from the issue text.