Deprecate include_relocations setting (elastic#47443)

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.

Author: DaveCTurner

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() {

server/src/test/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDeciderTests.java

@@ -635,7 +635,6 @@ public void testFreeDiskPercentageAfterShardAssigned() {
     public void testShardRelocationsTakenIntoAccount() {
         Settings diskSettings = Settings.builder()
                 .put(DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_DISK_THRESHOLD_ENABLED_SETTING.getKey(), true)
-                .put(DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_INCLUDE_RELOCATIONS_SETTING.getKey(), true)
                 .put(DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_LOW_DISK_WATERMARK_SETTING.getKey(), 0.7)
                 .put(DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_HIGH_DISK_WATERMARK_SETTING.getKey(), 0.8).build();
 
@@ -732,7 +731,6 @@ Settings.EMPTY, new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLU
     public void testCanRemainWithShardRelocatingAway() {
         Settings diskSettings = Settings.builder()
                 .put(DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_DISK_THRESHOLD_ENABLED_SETTING.getKey(), true)
-                .put(DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_INCLUDE_RELOCATIONS_SETTING.getKey(), true)
                 .put(DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_LOW_DISK_WATERMARK_SETTING.getKey(), "60%")
                 .put(DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_HIGH_DISK_WATERMARK_SETTING.getKey(), "70%").build();
 
@@ -862,7 +860,6 @@ Settings.EMPTY, new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLU
     public void testForSingleDataNode() {
         Settings diskSettings = Settings.builder()
                 .put(DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_DISK_THRESHOLD_ENABLED_SETTING.getKey(), true)
-                .put(DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_INCLUDE_RELOCATIONS_SETTING.getKey(), true)
                 .put(DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_LOW_DISK_WATERMARK_SETTING.getKey(), "60%")
                 .put(DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_HIGH_DISK_WATERMARK_SETTING.getKey(), "70%").build();