[SPARK-53770] Upgrade mockito to 5.20.0

Author: dongjoon-hyun

gradle/libs.versions.toml

@@ -25,7 +25,7 @@ log4j = "2.24.3"
 # Test
 junit = "5.13.4"
 jacoco = "0.8.13"
-mockito = "5.18.0"
+mockito = "5.20.0"
 
 # Build Analysis
 checkstyle = "10.23.1"