docs: add release notes for v5.6.6

Author: github-actions[bot]

release_notes/v5.6.6.md

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+## [5.6.6](https://github.com/NHSDigital/eps-common-workflows/compare/v5.6.5...v5.6.6) (2026-03-12)
+
+
+### Upgrade
+
+* [dependabot] - bump tar and npm (#86) ([383f3f9](https://github.com/NHSDigital/eps-common-workflows/commit/383f3f9eaf3cb553ebcd74897bfed4d5e387629e))
+
+## Info
+[Release workflow run](https://github.com/NHSDigital/eps-common-workflows/actions/runs/23004076858) - Workflow ID: 23004076858
+
+It was initialized by [eps-autoapprove-dependabot[bot]](https://github.com/apps/eps-autoapprove-dependabot)
+
+