docs: Add release notes for 2.9.0 (#5298)

* docs: Add release notes for 2.9.0

Workflow run: https://github.com/HumanSignal/label-studio-enterprise/actions/runs/7545621956



* Moved date and added helm chart version

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Co-authored-by: robot-ci-heartex <robot-ci-heartex@users.noreply.github.com>
Co-authored-by: Caitlin Wheeless <caitlin.wheeless@gmail.com>