[8.13](backport #38047) security: add permissions block to workflows

[mergify]

Details

⚠️ This PR was created by an automated tool. Please review the changes carefully. ⚠️

We want to set the default permissions for workflows to read-only for contents.
This is a security measure to prevent accidental changes to the repository.

This change adds a top-level permissions block to all workflows in the .github/workflows directory.

permissions:
  contents: read

In some cases workflows might need more permissions than just contents: read.
Please checkout this branch and add the necessary permissions to the workflows.

If your workflow uses a Personal Access Token (PAT), we can still add the permissions block,
but it will not have any effect.

Merging this PR as is might cause workflows that need more permissions to fail.

If there are any questions, please reach out to the @elastic/observablt-ci

---

This is an automatic backport of pull request #38047 done by [Mergify](https://mergify.com).

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[elasticmachine]

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• Start Time: 2024-03-27T08:56:07.145+0000

• Duration: 16 min 45 sec

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[mergify]

This pull request has not been merged yet. Could you please review and merge it @reakaleek? 🙏

[v1v]

I think we don't need to backport it