Docs(form-validations): Introduce migration guide for v1 refs #DS-800

packages/form-validations/MIGRATION-v1.md

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+# Migration Guide
+
+Introducing version 1 of the _spirit-form-validations_ package
+
+> Please follow these steps to safely upgrade your Design System to Spirit Design System v1 components.
+
+## Rename `validator_message` to `validation_text` [#DS-838](https://jira.lmc.cz/browse/DS-838) ([ba96ea6](https://github.com/lmc-eu/spirit-design-system/commit/ba96ea6))
+
+Instead of the `validator_message` value of the `data-spirit-element` attribute, use `validation_text`.
+
+- `<div data-spirit-element="validator_message">…</div>` → `<div data-spirit-element="validation_text">…</div>`
+
+---
+
+Please refer back to these instructions or reach out to our team if you encounter any issues during migration.