[Fleet client] Do not send Agent IDs in request bodies

[ycombinator]

What does this PR do?

This PR fixes the Fleet client, specifically for the Upgrade Agent and Un-enroll Agent API calls, to not send the Agent IDs in the request bodies of these APIs.

Why is it important?

For these API calls to work as expected. Needed by elastic/elastic-agent#2809.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md

[elasticmachine]

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• Start Time: 2023-06-08T18:31:12.564+0000

• Duration: 9 min 36 sec

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Test	Results
Failed	0
Passed	787
Skipped	11
Total	798

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