Deprecate include_relocations setting

Setting `cluster.routing.allocation.disk.include_relocations` to `false` is a
bad idea since it will lead to the kinds of overshoot that were otherwise fixed
in elastic#46079. This commit deprecates this setting so it can be removed in the next
major release.

docs/reference/modules/cluster/disk_allocator.asciidoc

@@ -69,6 +69,7 @@ PUT /twitter/_settings
 
 `cluster.routing.allocation.disk.include_relocations`::
 
+    deprecated[7.5] Future versions will always account for relocations.
     Defaults to +true+, which means that Elasticsearch will take into account
     shards that are currently being relocated to the target node when computing
     a node's disk usage. Taking relocating shards' sizes into account may,

server/src/main/java/org/elasticsearch/cluster/routing/allocation/DiskThresholdSettings.java

@@ -57,7 +57,7 @@ public class DiskThresholdSettings {
             Setting.Property.Dynamic, Setting.Property.NodeScope);
     public static final Setting<Boolean> CLUSTER_ROUTING_ALLOCATION_INCLUDE_RELOCATIONS_SETTING =
         Setting.boolSetting("cluster.routing.allocation.disk.include_relocations", true,
-            Setting.Property.Dynamic, Setting.Property.NodeScope);
+            Setting.Property.Dynamic, Setting.Property.NodeScope, Setting.Property.Deprecated);
     public static final Setting<TimeValue> CLUSTER_ROUTING_ALLOCATION_REROUTE_INTERVAL_SETTING =
         Setting.positiveTimeSetting("cluster.routing.allocation.disk.reroute_interval", TimeValue.timeValueSeconds(60),
             Setting.Property.Dynamic, Setting.Property.NodeScope);

server/src/test/java/org/elasticsearch/cluster/routing/allocation/DiskThresholdSettingsTests.java

@@ -72,6 +72,9 @@ public void testUpdate() {
         assertEquals(30L, diskThresholdSettings.getRerouteInterval().seconds());
         assertFalse(diskThresholdSettings.isEnabled());
         assertFalse(diskThresholdSettings.includeRelocations());
+
+        assertWarnings("[cluster.routing.allocation.disk.include_relocations] setting was deprecated in Elasticsearch and " +
+            "will be removed in a future release! See the breaking changes documentation for the next major version.");
     }
 
     public void testInvalidConstruction() {