fix(release): Remove mention of non published version of agent6

[chouetz]

What does this PR do?

Agent6 is not released since 6.53.0. Remove mentions of it in the changelog

Motivation

Clarity

Additional Notes

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes

[chouetz]

/merge

[dd-devflow]

🚂 MergeQueue: waiting for PR to be ready

This merge request is not mergeable yet, because of pending checks/missing approvals. It will be added to the queue as soon as checks pass and/or get approvals.
Note: if you pushed new commits since the last approval, you may need additional approval.
You can remove it from the waiting list with /remove command.

Use /merge -c to cancel this operation!

[agent-platform-auto-pr]

[Fast Unit Tests Report]

On pipeline 37575181 (CI Visibility). The following jobs did not run any unit tests:

Jobs:

• tests_deb-arm64-py3
• tests_deb-x64-py3
• tests_flavor_dogstatsd_deb-x64
• tests_flavor_heroku_deb-x64
• tests_flavor_iot_deb-x64
• tests_rpm-arm64-py3
• tests_rpm-x64-py3
• tests_windows-x64

If you modified Go files and expected unit tests to run in these jobs, please double check the job logs. If you think tests should have been executed reach out to #agent-devx-help

[dd-devflow]

🚂 MergeQueue: pull request added to the queue

The median merge time in main is 24m.

Use /merge -c to cancel this operation!