Adjust warning message for 8.x version in test (elastic#129980)

Deprecation warning messages are slightly different on the 8.* branch.
Somehow the warning message in this test was not covered by elastic#129628 .

Fixes elastic#129761

Author: albertzaharovits

muted-tests.yml

@@ -456,6 +456,3 @@ tests:
   - class: org.elasticsearch.index.engine.ThreadPoolMergeSchedulerTests
     method: testSchedulerCloseWaitsForRunningMerge
     issue: https://github.com/elastic/elasticsearch/issues/125236
-  - class: org.elasticsearch.index.engine.ThreadPoolMergeExecutorServiceDiskSpaceTests
-    method: testEnqueuedMergeTasksAreUnblockedWhenEstimatedMergeSizeChanges
-    issue: https://github.com/elastic/elasticsearch/issues/129761

server/src/test/java/org/elasticsearch/index/engine/ThreadPoolMergeExecutorServiceDiskSpaceTests.java

@@ -986,8 +986,7 @@ public void testEnqueuedMergeTasksAreUnblockedWhenEstimatedMergeSizeChanges() th
         }
         if (setThreadPoolMergeSchedulerSetting) {
             assertWarnings(
-                "[indices.merge.scheduler.use_thread_pool] setting was deprecated in Elasticsearch "
-                    + "and will be removed in a future release. See the breaking changes documentation for the next major version."
+                "[indices.merge.scheduler.use_thread_pool] setting was deprecated in Elasticsearch and will be removed in a future release."
             );
         }
     }