Added run task support for post plan run stage, removed pre apply

This commit stems from the change to make post plan the default run task stage, at the
time of this commit's writing! Since pre apply is under internal revision, we have removed
the block that polls the pre apply stage until the team decides to re-add support for pre apply
run tasks.

internal/cloud/backend_plan.go

@@ -346,6 +346,22 @@ in order to capture the filesystem context the remote workspace expects:
 	// status of the run will be "errored", but there is still policy
 	// information which should be shown.
 
+	// Await post-plan run tasks
+	integration := &IntegrationContext{
+		B:             b,
+		StopContext:   stopCtx,
+		CancelContext: cancelCtx,
+		Op:            op,
+		Run:           r,
+	}
+
+	if stageID := getTaskStageIDByName(r.TaskStages, tfe.PostPlan); stageID != nil {
+		err = b.runTasks(integration, integration.BeginOutput("Run Tasks (post-plan)"), *stageID)
+		if err != nil {
+			return r, err
+		}
+	}
+
 	// Show any cost estimation output.
 	if r.CostEstimate != nil {
 		err = b.costEstimate(stopCtx, cancelCtx, op, r)
@@ -362,28 +378,14 @@ in order to capture the filesystem context the remote workspace expects:
 		}
 	}
 
-	// Await pre-apply run tasks
-	if len(r.TaskStages) > 0 {
-		integration := &IntegrationContext{
-			B:             b,
-			StopContext:   stopCtx,
-			CancelContext: cancelCtx,
-			Op:            op,
-			Run:           r,
-		}
-
-		if stageID := getTaskStageIDByName(r.TaskStages, tfe.PreApply); stageID != nil {
-			err = b.runTasks(integration, integration.BeginOutput("Run Tasks (pre-apply)"), *stageID)
-			if err != nil {
-				return r, err
-			}
-		}
-	}
-
 	return r, nil
 }
 
 func getTaskStageIDByName(stages []*tfe.TaskStage, stageName tfe.Stage) *string {
+	if len(stages) == 0 {
+		return nil
+	}
+
 	for _, stage := range stages {
 		if stage.Stage == stageName {
 			return &stage.ID