Update 5.3-rc to alpine 3.21

Author: docker-library-bot

5.3-rc/Dockerfile

@@ -136,7 +136,7 @@
   },
   "5.3-rc": {
     "alpine": {
-      "version": "3.20"
+      "version": "3.21"
     },
     "version": "5.3-alpha"
   },