Update dependency org.mockito:mockito-core to v5.21.0

Author: renovate[bot]

gradle/libs.versions.toml

@@ -9,7 +9,7 @@ hivemq-testcontainer-junit5 = "2.0.0"
 jctools = "2.1.2"
 jetbrains-annotations = "26.0.2-1"
 junit-jupiter = "6.0.1"
-mockito = "5.20.0"
+mockito = "5.21.0"
 netty = "4.1.128.Final"
 paho = "1.2.5"
 pmd = "5.8.1"