Add PreAttack tactic to Sentinel scheduled alert rule

[NVolcz]

The missing PreAttack tactic was added to the azure-sdk-for-go v54.0.0.

[kaovd]

Thanks for the PR! Just suggesting a change to be in line with the schema for Attack tactics, elsewise this is invalid. Made this change and put PreAttack in the tests and looks all good.

> $ TF_ACC=1 go test -v -timeout=2h ./azurerm/internal/services/sentinel -run="TestAccSentinelAlertRuleScheduled_complete"           [±master ●]
2021/05/17 22:23:39 [DEBUG] not using binary driver name, it's no longer needed
2021/05/17 22:23:39 [DEBUG] not using binary driver name, it's no longer needed
=== RUN   TestAccSentinelAlertRuleScheduled_complete
=== PAUSE TestAccSentinelAlertRuleScheduled_complete
=== CONT  TestAccSentinelAlertRuleScheduled_complete
--- PASS: TestAccSentinelAlertRuleScheduled_complete (190.13s)
PASS
ok      github.com/terraform-providers/terraform-provider-azurerm/azurerm/internal/services/sentinel    190.255s

[katbyte]

Thanks @NVolcz - LGTM 👍

[ghost]

This has been released in version 2.60.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
    version = "~> 2.60.0"
}
# ... other configuration ...

[github-actions]

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