Update boot 2.6.1

- Migrate tests to junit5 and assertj as those
  are on a classpath automatically.
- Temporarily use spring.main.allow-circular-references=true
  to allow time for fixes to remove cycles.

spring-shell-jcommander-adapter/src/test/java/org/springframework/shell/jcommander/JCommanderParameterResolverTest.java

@@ -16,16 +16,12 @@
 
 package org.springframework.shell.jcommander;
 
-import static java.util.Arrays.asList;
-import static java.util.Collections.singletonList;
-import static org.assertj.core.api.Assertions.assertThat;
-
 import java.lang.reflect.Method;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.stream.Stream;
 
-import org.junit.Test;
+import org.junit.jupiter.api.Test;
 
 import org.springframework.core.MethodParameter;
 import org.springframework.shell.CompletionContext;
@@ -34,6 +30,10 @@ import org.springframework.shell.ParameterDescription;
 import org.springframework.shell.Utils;
 import org.springframework.util.ReflectionUtils;
 
+import static java.util.Arrays.asList;
+import static java.util.Collections.singletonList;
+import static org.assertj.core.api.Assertions.assertThat;
+
 /**
  * Unit test for {@link JCommanderParameterResolver}.
  *