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-samples/src/main/resources/application.yml

@@ -0,0 +1,3 @@
+spring:
+  main:
+    allow-circular-references: true