Slack return a 400 no_text when webhook called

Issue description

I'm trying to send Slack notifications on Gitlab events using gitlab webhooks and Slack apps webhooks Each time Gitlab trie to send a notification, it receive a 400 no_text

I've found a similar issue but it's closed: #205571 (closed)

How to reproduce

1. On Slack side, create an App, then add a webhook for this App.

I'm not using the Incoming webhooks Slack integration which is deprecated (https://slack.com/apps/A0F7XDUAZ-webhooks-entrants).

2. On Gitlab side, I went to my project page then Settings > Webhook.

3. Add a new Webhook

4. You can now test directly from the newly created Project hook

5. Each Slack answer is a 400 no_text

Additional informations

The example given by Slack is working perfectly with the same webhook

$> curl -X POST -H 'Content-type: application/json' --data '{"text":"Hello, World!"}' https://hooks.slack.com/services/[...]/[.....]/[..........]

ok%

I created a file containing the body payload sent by Gitlab to this slack webhook in the file payload.json then executed the same request

$> curl -X POST -H 'Content-type: application/json' --data @payload.json https://hooks.slack.com/services/[...]/[.....]/[..........]

no_text%

The payload.json file is attached to this issue payload.json

The Slack API probably changed when moving from legacy Slack incoming webhooks integration and Slack Apps webhook