add upgrade notes

Author: solverat

UPGRADE.md

@@ -3,6 +3,7 @@
 ## 5.3.4
 - **[SECURITY]** Introduce upload field reference and server-side MIME type validation. Read more about upload security [here](./docs/80_FileUpload.md#security)
 - **[SECURITY]** introduce policy validation for uploads. Read more about it [here](./docs/80_FileUpload.md#upload-policy-validator)
+- **[IMPROVEMENT]** Handle invalid runtime data JSON in DynamicFormType with proper exception [@hitsoul11](https://github.com/dachcom-digital/pimcore-formbuilder/pull/525)
 
 ## 5.3.3
 - **[BUGFIX]** Sanitize form field values by removing template tags during output transformation