chore(datacatalog): add information about deprecation of Data Catalog (#3979)

Co-authored-by: Jacek Spalinski <[email protected]>
Co-authored-by: Eric Schmidt <[email protected]>

Author: 3 people

datacatalog/README.md

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+**Data Catalog API deprecation**
+
+Data Catalog is deprecated and will be discontinued on January 30, 2026. For steps to transition your Data Catalog users, workloads, and content to Dataplex Catalog, see [Transition from Data Catalog to Dataplex Catalog](https://cloud.google.com/dataplex/docs/transition-to-dataplex-catalog).
+
+All API code samples under this folder are subject to decommissioning and will be removed after January 30, 2026.