[chore] add changelog entry for otel subcommand flag issue

[VihasMakwana]

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

[mergify]

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

• backport-./d./d is the label that automatically backports to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[elasticmachine]

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