[7.17] ci(slack-message): fix the error (backport #13493)

[mergify]

Motivation/summary

The reporting is not working but failing when sending the notification with the below error:

Error: Error: Need to provide valid JSON payload

I think there is a corner case when using : | ... and the json contains a multiline value...

channel-id: #apm-server
    payload: {
    "attachments": [
      {
        "pretext": "Workflow <https://github.com/elastic/apm-server/actions/runs/9649427993/attempts/1|benchmarks> triggered (`schedule`)",
        "color": "danger",
        "text": "Nightly APM Server benchmarks failed! SDH Duty assignee, please have a look and follow this <https://github.com/elastic/observability-dev/blob/main/docs/apm/apm-server/runbooks/benchmarks.md|Runbook>!
  ",
        "fields": [
          {
            "title": "Workflow",
            "short": true,
            "value": "<https://github.com/elastic/apm-server/actions/runs/9649427993/attempts/1|benchmarks #520>"
          },
          {
            "title": "Ref",
            "short": true,
            "value": "`main`"
          }
        ],
        "footer": "<https://github.com/elastic/apm-server|elastic/apm-server>",
        "footer_icon": "https://slack-imgs.com/?c=1&o1=wi32.he32.si&url=https%3A%2F%2Fslack.github.com%2Fstatic%2Fimg%2Ffavicon-neutral.png"
      }
    ]
  }

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Related issues

---

This is an automatic backport of pull request #13493 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of fd02bb0 has failed:

On branch mergify/bp/7.17/pr-13493
Your branch is up to date with 'origin/7.17'.

You are currently cherry-picking commit fd02bb051.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/workflows/benchmarks.yml

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally