docs: add release notes for v5.8.16

Author: github-actions[bot]

release_notes/v5.8.16.md

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+## [5.8.16](https://github.com/NHSDigital/eps-common-workflows/compare/v5.8.15...v5.8.16) (2026-03-27)
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+
+### Chore
+
+* [AEA-0000] - Dependabot cooldown (#115) ([211f854](https://github.com/NHSDigital/eps-common-workflows/commit/211f854a86de2f121ae9539e9695808fcadde45b))
+
+## Info
+[Release workflow run](https://github.com/NHSDigital/eps-common-workflows/actions/runs/23650412737) - Workflow ID: 23650412737
+
+It was initialized by [anthony-nhs](https://github.com/anthony-nhs)
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+