Rename the Snapshot Retention policy (#1284)

* Rename the Snapshot Retention policy

* Resolve comments

* Resolve comments

---------

Co-authored-by: Yufei Gu <yufei.apache.org>

Author: flyrain

polaris-core/src/main/resources/schemas/policies/system/snapshot-retention/2025-02-03.json renamed to polaris-core/src/main/resources/schemas/policies/system/snapshot-expiry/2025-02-03.json

@@ -1,8 +1,8 @@
 {
   "license": "Licensed under the Apache License, Version 2.0 (http://www.apache.org/licenses/LICENSE-2.0)",
-  "$id": "https://polaris.apache.org/schemas/policies/system/snapshot-retention/2025-02-03.json",
-  "title": "Snapshot Retention Policy",
-  "description": "Inheritable Polaris policy schema for Iceberg table snapshot retention",
+  "$id": "https://polaris.apache.org/schemas/policies/system/snapshot-expiry/2025-02-03.json",
+  "title": "Snapshot Expiry Policy",
+  "description": "Inheritable Polaris policy schema for managing Iceberg table snapshot expiry. This policy allows a table (or its parent) to indicate that snapshot expiry is enabled. It also defines how snapshots should be expired. Engines or clients can use this policy to safely remove expired snapshots and delete associated files, helping maintain a clean and efficient table state.",
   "type": "object",
   "properties": {
     "version": {
@@ -12,7 +12,7 @@
     },
     "enable": {
       "type": "boolean",
-      "description": "Enable or disable snapshot retention."
+      "description": "Enable or disable snapshot expiry."
     },
     "config": {
       "type": "object",