Remove "template" field in IndexTemplateMetaData (#42099)

Remove "template" field from XContent parsing in IndexTemplateMetaData

Author: PnPie

server/src/main/java/org/elasticsearch/cluster/metadata/IndexTemplateMetaData.java

@@ -20,7 +20,6 @@
 
 import com.carrotsearch.hppc.cursors.ObjectCursor;
 import com.carrotsearch.hppc.cursors.ObjectObjectCursor;
-import org.apache.logging.log4j.LogManager;
 import org.elasticsearch.ElasticsearchParseException;
 import org.elasticsearch.cluster.AbstractDiffable;
 import org.elasticsearch.cluster.Diff;
@@ -32,7 +31,6 @@
 import org.elasticsearch.common.compress.CompressedXContent;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
-import org.elasticsearch.common.logging.DeprecationLogger;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.util.set.Sets;
 import org.elasticsearch.common.xcontent.ToXContent;
@@ -52,8 +50,6 @@
 
 public class IndexTemplateMetaData extends AbstractDiffable<IndexTemplateMetaData> {
 
-    private static final DeprecationLogger deprecationLogger = new DeprecationLogger(LogManager.getLogger(IndexTemplateMetaData.class));
-
     private final String name;
 
     private final int order;
@@ -97,7 +93,7 @@ public IndexTemplateMetaData(String name, int order, Integer version,
         this.name = name;
         this.order = order;
         this.version = version;
-        this.patterns= patterns;
+        this.patterns = patterns;
         this.settings = settings;
         this.mappings = mappings;
         this.aliases = aliases;
@@ -227,7 +223,7 @@ public void writeTo(StreamOutput out) throws IOException {
     public static class Builder {
 
         private static final Set<String> VALID_FIELDS = Sets.newHashSet(
-            "template", "order", "mappings", "settings", "index_patterns", "aliases", "version");
+            "order", "mappings", "settings", "index_patterns", "aliases", "version");
 
         private String name;
 
@@ -487,11 +483,7 @@ public static IndexTemplateMetaData fromXContent(XContentParser parser, String t
                         builder.patterns(index_patterns);
                     }
                 } else if (token.isValue()) {
-                    // Prior to 5.1.0, elasticsearch only supported a single index pattern called `template` (#21009)
-                    if("template".equals(currentFieldName)) {
-                        deprecationLogger.deprecated("Deprecated field [template] used, replaced by [index_patterns]");
-                        builder.patterns(Collections.singletonList(parser.text()));
-                    } else if ("order".equals(currentFieldName)) {
+                    if ("order".equals(currentFieldName)) {
                         builder.order(parser.intValue());
                     } else if ("version".equals(currentFieldName)) {
                         builder.version(parser.intValue());