Add changelog fragment for legacy Certificate removal

Author: Jimbo4350

.changes/20260515_cardano_api_remove_legacy_certificate_type.yml

@@ -0,0 +1,16 @@
+project: cardano-api
+
+pr: 0
+
+kind:
+  - breaking
+
+description: |
+  Remove the deprecated `data Certificate era` GADT, its `ShelleyRelatedCertificate`/`ConwayCertificate`
+  constructors, all `make*Certificate` functions, all `*Requirements` GADTs, and the public
+  `Cardano.Api.Certificate` module. Consumers should use the certificate API exposed via
+  `Cardano.Api.Experimental.Certificate` (era-polymorphic, Conway onwards) or
+  `Cardano.Api.Compatible.Certificate` (covers all Shelley-based eras).
+  Pool-related types (`PoolId`, `StakePoolParameters`, `StakePoolRelay`, `StakePoolMetadataReference`)
+  and their conversion helpers remain in `Cardano.Api.Certificate.Internal` and are re-exported by
+  `Cardano.Api.Experimental.Certificate`.