Rename action META_OCCUPANCY->META

ably/specification@35bbeb7

Technically a breaking change. We're not considering it such because
`Message.action` was only introduced recently as part of the
annotations/materialization work, which isn't yet part of a publicly
released feature yet, and isn't added to our [main
documentation](https://ably.com/docs/api/realtime-sdk/types#message). So
it's just very unlikely that anyone is relying on the action yet. Our
inband occupancy events docs still all say to use the message name,
which is not changing.

Author: SimonWoolf

lib/src/main/java/io/ably/lib/types/MessageAction.java

@@ -4,7 +4,7 @@ public enum MessageAction {
     MESSAGE_CREATE, // 0
     MESSAGE_UPDATE, // 1
     MESSAGE_DELETE, // 2
-    META_OCCUPANCY, // 3
+    META, // 3
     MESSAGE_SUMMARY; // 4
 
     static MessageAction tryFindByOrdinal(int ordinal) {