[8.x] Remove index.merge.policy.max_merge_at_once_explicit deprecation (#124792)

This setting is not getting removed in ES 9.0, so its usage should not generate a critical deprecation warning.
This is an index setting and should have been marked as `IndexSettingDeprecatedInV8AndRemovedInV9`, but we can't
change that in 8.x and will mark it as `IndexSettingDeprecatedInV9AndRemovedInV10` in 9.0

See #90264
See ES-11224

Author: arteam

server/src/main/java/org/elasticsearch/index/MergePolicyConfig.java

@@ -236,7 +236,7 @@ MergePolicy getMergePolicy(MergePolicyConfig config, boolean isTimeBasedIndex) {
         "index.merge.policy.max_merge_at_once_explicit",
         30,
         2,
-        Property.Deprecated, // When removing in 9.0 follow the approach of IndexSettingDeprecatedInV7AndRemovedInV8
+        // When removing in 9.0 follow the approach of IndexSettingDeprecatedInV7AndRemovedInV8
         Property.Dynamic,
         Property.IndexScope
     );