Use AssertJ's fail, not JUnit's

This commit also updates Checkstyle to prevent use of JUnit's
assertions from being reintroduced.

See gh-37655

src/checkstyle/checkstyle.xml

@@ -32,7 +32,7 @@
 		</module>
 		<module name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
 			<property name="maximum" value="0"/>
-			<property name="format" value="org\.junit\.Assert\.assert" />
+			<property name="format" value="org\.junit\.Assert|org\.junit\.jupiter\.api\.Assertions" />
 			<property name="message"
 				value="Please use AssertJ imports." />
 			<property name="ignoreComments" value="true" />