[Fix] Reflect backend updates in state for databricks_app (#4337)

## Changes
<!-- Summary of your changes that are easy to understand -->
Following #4099, updates to Databricks Apps only reflected the ones in
the plan, and not the ones coming back from the backend call to update
the app. This PR includes those changes in.

## Tests
<!-- 
How is this tested? Please see the checklist below and also describe any
other relevant tests
-->

- [x] `make test` run locally
- [ ] relevant change in `docs/` folder
- [x] covered with integration tests in `internal/acceptance`
- [x] using Go SDK
- [x] using TF Plugin Framework

Co-authored-by: Omer Lachish <rauchy@users.noreply.github.com>

internal/providers/pluginfw/products/app/resource_app.go

@@ -159,15 +159,15 @@ func (a *resourceApp) Update(ctx context.Context, req resource.UpdateRequest, re
 	if resp.Diagnostics.HasError() {
 		return
 	}
-	_, err := w.Apps.Update(ctx, apps.UpdateAppRequest{App: &appGoSdk, Name: app.Name.ValueString()})
+	response, err := w.Apps.Update(ctx, apps.UpdateAppRequest{App: &appGoSdk, Name: app.Name.ValueString()})
 	if err != nil {
 		resp.Diagnostics.AddError("failed to update app", err.Error())
 		return
 	}
 
 	// Store the updated version of the app in state
 	var newApp apps_tf.App
-	resp.Diagnostics.Append(converters.GoSdkToTfSdkStruct(ctx, appGoSdk, &newApp)...)
+	resp.Diagnostics.Append(converters.GoSdkToTfSdkStruct(ctx, response, &newApp)...)
 	if resp.Diagnostics.HasError() {
 		return
 	}

internal/providers/pluginfw/products/app/resource_app_acc_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 
 	"github.com/databricks/terraform-provider-databricks/internal/acceptance"
+	"github.com/hashicorp/terraform-plugin-testing/terraform"
+	"github.com/stretchr/testify/assert"
 )
 
 const baseResources = `
@@ -118,14 +120,24 @@ is required`)),
 }
 
 func TestAccAppResource(t *testing.T) {
+	var updateTime string
 	acceptance.LoadWorkspaceEnv(t)
 	if acceptance.IsGcp(t) {
 		acceptance.Skipf(t)("not available on GCP")
 	}
 	acceptance.WorkspaceLevel(t, acceptance.Step{
 		Template: makeTemplate("My app"),
+		Check: func(s *terraform.State) error {
+			updateTime = s.RootModule().Resources["databricks_app.this"].Primary.Attributes["update_time"]
+			return nil
+		},
 	}, acceptance.Step{
 		Template: makeTemplate("My new app"),
+		Check: func(s *terraform.State) error {
+			var newUpdateTime = s.RootModule().Resources["databricks_app.this"].Primary.Attributes["update_time"]
+			assert.NotEqual(t, updateTime, newUpdateTime)
+			return nil
+		},
 	}, acceptance.Step{
 		ImportState:       true,
 		ResourceName:      "databricks_app.this",