docs: add release notes for v5.8.21

Author: github-actions[bot]

release_notes/v5.8.21.md

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+## [5.8.21](https://github.com/NHSDigital/eps-common-workflows/compare/v5.8.20...v5.8.21) (2026-04-01)
+
+
+### Upgrade
+
+* [dependabot] - sync Copilot instructions (#123) ([6aa0796](https://github.com/NHSDigital/eps-common-workflows/commit/6aa079639cafd26a6776974521a1bb98aa44226c))
+
+## Info
+[Release workflow run](https://github.com/NHSDigital/eps-common-workflows/actions/runs/23852974090) - Workflow ID: 23852974090
+
+It was initialized by [anthony-nhs](https://github.com/anthony-nhs)
+
+