Guard tests for $lookup with let & pipeline

Add guard to skip tests prior to 5.0 server version.

Related to: #3322

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationTests.java

@@ -1520,6 +1520,7 @@ public class AggregationTests {
 	}
 
 	@Test // GH-3322
+	@EnableIfMongoServerVersion(isGreaterThanEqual = "5.0")
 	void shouldLookupPeopleCorrectlyWithPipeline() {
 		createUsersWithReferencedPersons();
 
@@ -1538,6 +1539,7 @@ public class AggregationTests {
 	}
 
 	@Test // GH-3322
+	@EnableIfMongoServerVersion(isGreaterThanEqual = "5.0")
 	void shouldLookupPeopleCorrectlyWithPipelineAndLet() {
 		createUsersWithReferencedPersons();