[Java] Upgrade to Mockito 5.21.0.

Author: vyazelenko

gradle/libs.versions.toml

@@ -9,7 +9,7 @@ jqwik = "1.9.3"
 jmh = "1.37"
 json = "20250517"
 junit = "6.0.1"
-mockito = "5.20.0"
+mockito = "5.21.0"
 plexus = "3.3.0"
 shadow = "9.3.0"
 versions = "0.53.0"