Update dependency org.assertj:assertj-core to v3.21.0

Author: renovate-bot

build.gradle

@@ -31,7 +31,7 @@ apply from: "$rootDir/gradle-scripts/set-library-version.gradle"
 ext {
     mockitoVersion = '3.12.4'
     slf4jTestVersion = '1.2.0'
-    assertjVersion = '3.20.2'
+    assertjVersion = '3.21.0'
     pmdVersion = '5.6.1'
     jacocoVersion = '0.8.6'
     findbugsVersion = '3.0.1'