Updates to latest build and latest Brave

[codefromthecrypt]

This migrates all tests and fixtures from JUnit 4.x to Jupiter. Notably, this allows use of latest brave 5.18.

After manually converting the build to latest, I began porting by using this command from @TeamModerne, which got surprisingly far despite this being a very complex project.

./mvnw org.openrewrite.maven:rewrite-maven-plugin:run \
  -Drewrite.recipeArtifactCoordinates=org.openrewrite.recipe:rewrite-testing-frameworks:LATEST \
  -Drewrite.activeRecipes=org.openrewrite.java.testing.junit5.JUnit4to5Migration

After that, I focused on the following areas which led to conclusion:

• fix formatting and visibility
• revert mockwebserver3 upgrade as it introduced classpath incompatibility
• port internal rules to extensions
• polish to use testcontainers jupiter