fix(webhook): clarify endpoint expects application/json payloads (#13786)

This PR updates the subtitle under the "Endpoint URL" input on the
webhook creation form.

The updated text clearly states that the server sends POST requests with
application/json and recommends setting the correct Content-Type.

This addresses issue #13757.

Closes #13757

---------

Co-authored-by: Charles Bochet <charlesBochet@users.noreply.github.com>
Co-authored-by: ehconitin <nitinkoche03@gmail.com>

packages/twenty-front/src/pages/settings/developers/__stories__/webhooks/SettingsDevelopersWebhookNew.stories.tsx

@@ -29,7 +29,7 @@ export const Default: Story = {
     const canvas = within(canvasElement);
     await canvas.findByText('New Webhook', undefined, { timeout: 3000 });
     await canvas.findByText(
-      'We will send POST requests to this endpoint for every new event',
+      'We will send a POST request to this endpoint for each new event in application/json format',
     );
     await canvas.findByPlaceholderText('https://example.com/webhook');
   },