[9.3] (backport #12187) [chore] add changelog entry for otel subcommand flag issue

[mergify]

What does this PR do?

After Blake's changes, the otel subcommand is no longer embedded and it doesn't support the elastic-agent specific flags. As a result, it errors out if any of the agent-specific flags are used.

This PR adds a change log entry for the same.

Why is it important?

Because this is a breaking change.

Checklist

• I have read and understood the pull request guidelines of this project.
• 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/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

Following command will fail in 9.3 and subsequent releases:

./elastic-agent -c ./otel.yml otel

Related issues

• Closes [9.3] Elastic Agent command line flags not working for otel sub-command #12070

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  This is an automatic backport of pull request [chore] add changelog entry for otel subcommand flag issue #12187 done by Mergify.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)