Update mockito monorepo to v5.21.0

Author: renovate[bot]

gradle/libs.versions.toml

@@ -8,7 +8,7 @@ jgitver = '0.10.0-rc03'
 jspecify = '1.0.0'
 junit = '6.0.1'
 assertj = '3.27.6'
-mockito = '5.20.0'
+mockito = '5.21.0'
 
 slf4j = '2.1.0-alpha1'
 logback = '1.5.21'