security: add permissions block to workflows

[elastic-vault-github-plugin-prod]

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

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[mergify]

This pull request does not have a backport label. Could you fix it @elastic-vault-github-plugin-prod[bot]? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8.19 is the label to automatically backport to the 8.19 branch.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
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• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

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