ParentAggregation tests only valid in Elasticsearch 6.6.0+

This commit limits ParentAggregation tests to run only when
targeting Elasticsearch 6.6.0+. Add documentation that highlights this.

(cherry picked from commit 6938f0f)

Author: russcam

src/Nest/Aggregations/AggregationContainer.cs

@@ -189,6 +189,7 @@ public interface IAggregationContainer
 		[DataMember(Name = "nested")]
 		INestedAggregation Nested { get; set; }
 
+		/// <inheritdoc cref="IParentAggregation"/>
 		[DataMember(Name = "parent")]
 		IParentAggregation Parent { get; set; }
 
@@ -319,6 +320,7 @@ public class AggregationContainer : IAggregationContainer
 
 		public INestedAggregation Nested { get; set; }
 
+		/// <inheritdoc cref="IParentAggregation"/>
 		public IParentAggregation Parent { get; set; }
 
 		public IPercentileRanksAggregation PercentileRanks { get; set; }
@@ -601,6 +603,7 @@ Func<NestedAggregationDescriptor<T>, INestedAggregation> selector
 		) =>
 			_SetInnerAggregation(name, selector, (a, d) => a.Nested = d);
 
+		/// <inheritdoc cref="IParentAggregation"/>
 		public AggregationContainerDescriptor<T> Parent<TParent>(string name,
 			Func<ParentAggregationDescriptor<T, TParent>, IParentAggregation> selector
 		) where TParent : class =>

src/Nest/Aggregations/Bucket/Parent/ParentAggregation.cs

@@ -3,6 +3,13 @@
 
 namespace Nest
 {
+	/// <summary>
+	/// A special single bucket aggregation that selects parent documents that
+	/// have the specified type, as defined in a join field.
+	/// </summary>
+	/// <remarks>
+	/// Valid only in Elasticsearch 6.6.0+
+	/// </remarks>
 	[InterfaceDataContract]
 	[ReadAs(typeof(ParentAggregation))]
 	public interface IParentAggregation : IBucketAggregation
@@ -14,6 +21,7 @@ public interface IParentAggregation : IBucketAggregation
 		RelationName Type { get; set; }
 	}
 
+	/// <inheritdoc cref="IParentAggregation"/>
 	public class ParentAggregation : BucketAggregationBase, IParentAggregation
 	{
 		internal ParentAggregation() { }
@@ -25,6 +33,7 @@ internal ParentAggregation() { }
 		internal override void WrapInContainer(AggregationContainer c) => c.Parent = this;
 	}
 
+	/// <inheritdoc cref="IParentAggregation"/>
 	public class ParentAggregationDescriptor<T, TParent>
 		: BucketAggregationDescriptorBase<ParentAggregationDescriptor<T, TParent>, IParentAggregation, TParent>, IParentAggregation
 		where T : class

src/Tests/Tests/Aggregations/Bucket/Parent/ParentAggregationUsageTests.cs

@@ -20,6 +20,7 @@ namespace Tests.Aggregations.Bucket.Parent
 	 *
 	 * Be sure to read the Elasticsearch documentation on {ref_current}/search-aggregations-bucket-parent-aggregation.html[Parent Aggregation].
 	 */
+	[SkipVersion("<6.6.0", "Parent aggregation only available in Elasticsearch 6.6.0+")]
 	public class ParentAggregationUsageTests : ApiIntegrationTestBase<ReadOnlyCluster, ISearchResponse<CommitActivity>, ISearchRequest, SearchDescriptor<CommitActivity>, SearchRequest<CommitActivity>>
 	{
 		public ParentAggregationUsageTests(ReadOnlyCluster i, EndpointUsage usage) : base(i, usage) { }