Bump assertj-core from 3.14.0 to 3.15.0 (#1390)

Bumps [assertj-core](https://github.com/joel-costigliola/assertj-core) from 3.14.0 to 3.15.0.
- [Release notes](https://github.com/joel-costigliola/assertj-core/releases)
- [Commits](assertj/assertj@assertj-core-3.14.0...assertj-core-3.15.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

exonum-java-binding/pom.xml

@@ -107,7 +107,7 @@
     <!-- Empty for all JDKs but 11 -->
     <maven.javadoc.joption></maven.javadoc.joption>
 
-    <assertj.version>3.14.0</assertj.version>
+    <assertj.version>3.15.0</assertj.version>
     <checkstyle.configLocation>${project.basedir}/../checkstyle.xml</checkstyle.configLocation>
     <checkstyle.severity>warning</checkstyle.severity>
     <guice.version>4.2.2</guice.version>