Update AssertJ recipes to current recipe code style

[jevanlingen]

What's changed?

The AssertJ recipes use an older recipe code style; replaced all of 'm with an anonymous class + remove boilerplate code. The class JUnitAssertInstanceOfToAssertThat (created in #606) i is used as the base template for all changed recipes.

What's your motivation?

Code convergence

Any additional context

Recipes in other packages I have left out of the scope of this PR. For example, the cleanup recipes also use the “old” recipe style.

Checklist

• I've added unit tests to cover both positive and negative cases
• I've read and applied the recipe conventions and best practices
• I've used the IntelliJ IDEA auto-formatter on affected files

[github-actions]

Some suggestions could not be made:

• src/main/java/org/openrewrite/java/testing/assertj/JUnitAssertFalseToAssertThat.java
  • lines 27-27
• src/main/java/org/openrewrite/java/testing/assertj/JUnitAssertNotEqualsToAssertThat.java
  • lines 27-27
• src/main/java/org/openrewrite/java/testing/assertj/JUnitAssertNotNullToAssertThat.java
  • lines 27-27
• src/main/java/org/openrewrite/java/testing/assertj/JUnitAssertNullToAssertThat.java
  • lines 27-27
• src/main/java/org/openrewrite/java/testing/assertj/JUnitAssertSameToAssertThat.java
  • lines 27-27
• src/main/java/org/openrewrite/java/testing/assertj/JUnitAssertTrueToAssertThat.java
  • lines 27-27

[timtebeek]

Great to see this initial work already; helpful to have some examples to point out best practices (as opposed to what you were seeing). I've left a few more comments on patterns we can clear out; please apply those not just where I've left my remarks, but anywhere in these affected files.

[timtebeek]

Great simplification, thanks a lot! Should help to propagate the proper patterns.