Change skip_unavailable default value to true (#105792)

In order to improve the experience of cross-cluster search, we are changing
the default value of the remote cluster `skip_unavailable` setting from `false` to `true`.

This setting causes any cross-cluster _search (or _async_search) to entirely fail when
any remote cluster with `skip_unavailable=false` is either unavailable (connection to it fails)
or if the search on it fails on all shards.

Setting `skip_unavailable=true` allows partial results from other clusters to be
returned. In that case, the search response cluster metadata will show a `skipped`
status, so the user can see that no data came in from that cluster. Kibana also
now leverages this metadata in the cross-cluster search responses to allow users
to see how many clusters returned data and drill down into which clusters did not
(including failure messages).

Currently, the user/admin has to specifically set the value to `true` in the configs, like so:

```
cluster:
    remote:
        remote1:
            seeds: 10.10.10.10:9300
            skip_unavailable: true
```

even though that is probably what search admins want in the vast majority of cases.

Setting `skip_unavailable=false` should be a conscious (and probably rare) choice
by an Elasticsearch admin that a particular cluster's results are so essential to a
search (or visualization in dashboard or Discover panel) that no results at all should
be shown if it cannot return any results.

docs/reference/ccr/getting-started.asciidoc

@@ -147,7 +147,7 @@ cluster with cluster alias `leader`.
     "num_nodes_connected" : 1, <1>
     "max_connections_per_cluster" : 3,
     "initial_connect_timeout" : "30s",
-    "skip_unavailable" : false,
+    "skip_unavailable" : true,
     "mode" : "sniff"
   }
 }