Remove version dependent logic from CompletionFieldMapper

We have version based logic that applies the limit to number of completion contexts
only to indices created from 8.0 on. Those are the only indices we can now have
in a cluster, hence we can remove the version based conditional.

Relates to #38675

Author: javanna

server/src/main/java/org/elasticsearch/index/mapper/CompletionFieldMapper.java

@@ -19,12 +19,10 @@
 import org.apache.lucene.search.suggest.document.RegexCompletionQuery;
 import org.apache.lucene.search.suggest.document.SuggestField;
 import org.elasticsearch.common.ParsingException;
-import org.elasticsearch.common.logging.DeprecationCategory;
 import org.elasticsearch.common.logging.DeprecationLogger;
 import org.elasticsearch.common.unit.Fuzziness;
 import org.elasticsearch.common.util.Maps;
 import org.elasticsearch.index.IndexVersion;
-import org.elasticsearch.index.IndexVersions;
 import org.elasticsearch.index.analysis.AnalyzerScope;
 import org.elasticsearch.index.analysis.NamedAnalyzer;
 import org.elasticsearch.index.query.SearchExecutionContext;
@@ -213,29 +211,11 @@ public CompletionFieldMapper build(MapperBuilderContext context) {
 
         private void checkCompletionContextsLimit() {
             if (this.contexts.getValue() != null && this.contexts.getValue().size() > COMPLETION_CONTEXTS_LIMIT) {
-                if (indexVersionCreated.onOrAfter(IndexVersions.V_8_0_0)) {
-                    throw new IllegalArgumentException(
-                        "Limit of completion field contexts [" + COMPLETION_CONTEXTS_LIMIT + "] has been exceeded"
-                    );
-                } else {
-                    deprecationLogger.warn(
-                        DeprecationCategory.MAPPINGS,
-                        "excessive_completion_contexts",
-                        "You have defined more than ["
-                            + COMPLETION_CONTEXTS_LIMIT
-                            + "] completion contexts"
-                            + " in the mapping for field ["
-                            + leafName()
-                            + "]. "
-                            + "The maximum allowed number of completion contexts in a mapping will be limited to "
-                            + "["
-                            + COMPLETION_CONTEXTS_LIMIT
-                            + "] starting in version [8.0]."
-                    );
-                }
+                throw new IllegalArgumentException(
+                    "Limit of completion field contexts [" + COMPLETION_CONTEXTS_LIMIT + "] has been exceeded"
+                );
             }
         }
-
     }
 
     public static final Set<String> ALLOWED_CONTENT_FIELD_NAMES = Set.of(