[7.x] [DOCS] Update api_key example on elasticsearch output

[ugosan]

What does this PR do?

Update api_key example on elasticsearch output to include the id and the api_key

Why is it important?

Current example is misleading as the elasticsearch output uses api_key to refer to what should be id:api_key. A few customers have stumbled on this as they thought they would need only the api_key part from the POST /_security/api_key response:

{
  "id" : "ZCV7VnwBgnX0T19fN8Qe",
  "name" : "my-api-key",
  "expiration" : 1633624888071,
  "api_key" : "sWBxrOMISVCmEyGDlwwcOg"
}

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[mergify]

This pull request does not have a backport label. Could you fix it @ugosan? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

Pinging @elastic/obs-docs (Team:Docs)

[elasticmachine]

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• Start Time: 2021-10-06T18:01:41.850+0000

• Duration: 23 min 10 sec

• Commit: df4d678

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[bmorelli25]

Thank you!