Update JUnit from version 4 to 5

JUnit 4 is discontinued and JUnit 6 cannot be used because it requires
Java 17, but Fake SFTP Server Lambda's minimal Java version is 8. That
is why JUnit 5 has been chosen.

Author: stefanbirkner

pom.xml

@@ -64,18 +64,18 @@
             <version>[2.8.0]</version>
             <scope>test</scope>
         </dependency>
-        <dependency>
-            <groupId>junit</groupId>
-            <artifactId>junit</artifactId>
-            <version>[4.13.2]</version>
-            <scope>test</scope>
-        </dependency>
         <dependency>
             <groupId>org.assertj</groupId>
             <artifactId>assertj-core</artifactId>
             <version>[3.19.0]</version>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+            <version>[5.14.2]</version>
+            <scope>test</scope>
+        </dependency>
         <dependency>
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-nop</artifactId>