Upgrade to Assertj 3.9.0

Closes gh-11552

spring-boot-project/spring-boot-dependencies/pom.xml

@@ -29,7 +29,7 @@
 		<appengine-sdk.version>1.9.60</appengine-sdk.version>
 		<artemis.version>2.4.0</artemis.version>
 		<aspectj.version>1.8.13</aspectj.version>
-		<assertj.version>3.8.0</assertj.version>
+		<assertj.version>3.9.0</assertj.version>
 		<atomikos.version>4.0.5</atomikos.version>
 		<bitronix.version>2.1.4</bitronix.version>
 		<byte-buddy.version>1.7.9</byte-buddy.version>