Rename dangling index APIs (#58266)

The dangling_indices.import API name could cause issues in the client
libs because import is a reserved word in many languages. Rename the
API to avoid this, and rename the other APIs for consistency.

Related to #48366.

client/rest-high-level/src/test/java/org/elasticsearch/client/RestHighLevelClientTests.java

@@ -813,9 +813,9 @@ public class RestHighLevelClientTests extends ESTestCase {
             "cluster.stats",
             "cluster.post_voting_config_exclusions",
             "cluster.delete_voting_config_exclusions",
-            "dangling_indices.delete",
-            "dangling_indices.import",
-            "dangling_indices.list",
+            "dangling_indices.delete_dangling_index",
+            "dangling_indices.import_dangling_index",
+            "dangling_indices.list_dangling_indices",
             "indices.shard_stores",
             "indices.upgrade",
             "indices.recovery",

rest-api-spec/src/main/resources/rest-api-spec/api/dangling_indices.delete.json → rest-api-spec/src/main/resources/rest-api-spec/api/dangling_indices.delete_dangling_index.json

@@ -1,5 +1,5 @@
 {
-  "dangling_indices.delete": {
+  "dangling_indices.delete_dangling_index": {
     "documentation": {
       "url": "https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-gateway-dangling-indices.html",
       "description": "Deletes the specified dangling index"

rest-api-spec/src/main/resources/rest-api-spec/api/dangling_indices.import.json → rest-api-spec/src/main/resources/rest-api-spec/api/dangling_indices.import_dangling_index.json

@@ -1,5 +1,5 @@
 {
-  "dangling_indices.import": {
+  "dangling_indices.import_dangling_index": {
     "documentation": {
       "url": "https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-gateway-dangling-indices.html",
       "description": "Imports the specified dangling index"

rest-api-spec/src/main/resources/rest-api-spec/api/dangling_indices.list.json → rest-api-spec/src/main/resources/rest-api-spec/api/dangling_indices.list_dangling_indices.json

@@ -1,5 +1,5 @@
 {
-  "dangling_indices.list": {
+  "dangling_indices.list_dangling_indices": {
     "documentation": {
       "url": "https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-gateway-dangling-indices.html",
       "description": "Returns all dangling indices."