Enhance deprecation message when index name contains ':' (#34867)

The message was a little unclear in that we will still support indices that have
a ':' in the name in 7.0, we will only prevent index creation containing a ':'

Relates to #23892 (comment)

Author: dakrone

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

@@ -169,7 +169,8 @@ public static void validateIndexOrAliasName(String index, BiFunction<String, Str
         }
         if (index.contains(":")) {
             deprecationLogger.deprecated("index or alias name [" + index +
-                            "] containing ':' is deprecated and will not be supported in Elasticsearch 7.0+");
+                            "] containing ':' is deprecated. Elasticsearch 7.x will read, " +
+                            "but not allow creation of new indices containing ':'");
         }
         if (index.charAt(0) == '_' || index.charAt(0) == '-' || index.charAt(0) == '+') {
             throw exceptionCtor.apply(index, "must not start with '_', '-', or '+'");

server/src/test/java/org/elasticsearch/cluster/metadata/MetaDataCreateIndexServiceTests.java

@@ -415,7 +415,8 @@ public void testValidateIndexName() throws Exception {
 
         MetaDataCreateIndexService.validateIndexName("foo:bar", ClusterState.builder(ClusterName.CLUSTER_NAME_SETTING
                         .getDefault(Settings.EMPTY)).build());
-        assertWarnings("index or alias name [foo:bar] containing ':' is deprecated and will not be supported in Elasticsearch 7.0+");
+        assertWarnings("index or alias name [foo:bar] containing ':' is deprecated. Elasticsearch 7.x will read, " +
+                        "but not allow creation of new indices containing ':'");
     }
 
     private void validateIndexName(String indexName, String errorMessage) {