`azurerm_app_service_connection`: changing default behavior of `vnet_solution`

[jiaweitao001]

• The original behavior was set vnet_solution to VNetSolutionTypePrivateLink. Now it's changed to null by service team's request.

• All related tests are passing.

--- PASS: TestAccServiceConnectorAppServiceCosmosdb_basic (978.20s)
--- PASS: TestAccServiceConnectorAppService_complete (1026.74s)
--- PASS: TestAccServiceConnectorSpringCloudCosmosdb_basic (1035.89s)
--- PASS: TestAccServiceConnectorSpringCloud_complete (1146.73s)
--- PASS: TestAccServiceConnectorAppServiceCosmosdb_update (1221.24s)
--- PASS: TestAccServiceConnectorSpringCloudCosmosdb_update (1885.30s)
PASS
ok      github.com/hashicorp/terraform-provider-azurerm/internal/services/serviceconnector      1899.145s