milvus default to v2.4.15, bump version #571

Job	Run time
System Integration Test (ubuntu18.04)	11m 45s
System Integration Test (ubuntu18.04)	11m 45s
Code Check (ubuntu18.04)	5m 17s
Code Check (ubuntu18.04)	5m 17s
System Integration Test for Mixture (ubuntu18.04)	3m 34s
System Integration Test for Mixture (ubuntu18.04)	9m 30s
System Integration Test for New Feature (ubuntu18.04)	14m 48s
System Integration Test for New Feature (ubuntu18.04)	14m 48s
Unit Test (ubuntu18.04)	6m 9s
Unit Test (ubuntu18.04)	6m 9s
System Integration Test for Milvus Upgrade (ubuntu18.04)	3m 30s
System Integration Test for Milvus Upgrade (ubuntu18.04)	21m 21s
System Integration Test for Upgrade (ubuntu18.04)	11m 48s
System Integration Test for Upgrade (ubuntu18.04)	11m 48s
	2h 17m 29s

Provide feedback