resource/aws_ssm_maintenance_window_task: Mark service_role_arn as Co…

…mputed, add CHANGELOG for #12200

Output from acceptance testing in AWS Commercial:

```
--- PASS: TestAccAWSSSMMaintenanceWindowTask_disappears (21.36s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_emptyNotificationConfig (22.49s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_noRole (23.79s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_TaskInvocationStepFunctionParameters (25.18s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_updateForcesNewResource (37.93s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_basic (38.21s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_TaskInvocationAutomationParameters (46.55s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_TaskInvocationRunCommandParameters (48.17s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_TaskInvocationLambdaParameters (48.58s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_TaskInvocationRunCommandParametersCloudWatch (49.26s)
```

Output from acceptance testing in AWS GovCloud (US):

```
--- PASS: TestAccAWSSSMMaintenanceWindowTask_noRole (26.04s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_disappears (26.62s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_emptyNotificationConfig (27.01s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_TaskInvocationStepFunctionParameters (29.35s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_updateForcesNewResource (43.07s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_basic (43.41s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_TaskInvocationLambdaParameters (46.46s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_TaskInvocationAutomationParameters (51.63s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_TaskInvocationRunCommandParameters (51.98s)
--- PASS: TestAccAWSSSMMaintenanceWindowTask_TaskInvocationRunCommandParametersCloudWatch (59.12s)
```

.changelog/12200.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+resource/aws_ssm_maintenance_window_task: Make `service_role_arn` optional
+```

aws/resource_aws_ssm_maintenance_window_task.go

@@ -57,6 +57,7 @@ func resourceAwsSsmMaintenanceWindowTask() *schema.Resource {
 			"service_role_arn": {
 				Type:         schema.TypeString,
 				Optional:     true,
+				Computed:     true,
 				ValidateFunc: validateArn,
 			},
 

aws/resource_aws_ssm_maintenance_window_task_test.go

@@ -66,7 +66,6 @@ func TestAccAWSSSMMaintenanceWindowTask_noRole(t *testing.T) {
 				Check: resource.ComposeTestCheckFunc(
 					testAccCheckAWSSSMMaintenanceWindowTaskExists(resourceName, &task),
 				),
-				ExpectNonEmptyPlan: true,
 			},
 		},
 	})
@@ -649,16 +648,16 @@ resource "aws_ssm_maintenance_window_task" "test" {
 }
 
 func testAccAWSSSMMaintenanceWindowTaskNoRoleConfig(rName string) string {
-	return fmt.Sprintf(testAccAWSSSMMaintenanceWindowTaskConfigBase(rName)+`
+	return fmt.Sprintf(testAccAWSSSMMaintenanceWindowTaskConfigBase(rName) + `
 resource "aws_ssm_maintenance_window_task" "test" {
-  window_id       = aws_ssm_maintenance_window.test.id
-  task_type       = "RUN_COMMAND"
-  task_arn        = "AWS-RunShellScript"
-  priority        = 1
-  max_concurrency = "2"
-  max_errors      = "1"
+  description     = "This resource is for test purpose only"
+  max_concurrency = 2
+  max_errors      = 1
   name            = "TestMaintenanceWindowTask"
-  description     = "This resource is for test purpose only"  
+  priority        = 1
+  task_arn        = "AWS-RunShellScript"
+  task_type       = "RUN_COMMAND"
+  window_id       = aws_ssm_maintenance_window.test.id
 
   targets {
     key    = "WindowTargetIds"