[Subtask] Upgrade the test toolchain: remove JUnit 4, JUnit 6, Testcontainers 2 #14105

Describe the subtask

Move the test toolchain to the versions the container framework (#14002) builds on. Each PR merges on main on its own:

1. Remove JUnit 4 and declare what tests use. Each module declares the test libraries it imports instead of getting them transitively, and a checkstyle IllegalImport rule bans org.junit.* outside jupiter and platform. 42 test files still use org.junit.Assert, and two JUnit 4 test classes remain: TestOwnerDetails (clients/cli) never ran, since that module has no vintage engine; TestGravitinoCatalogStore ran through the vintage engine flink-test-utils brings. They compile only because Testcontainers 1.x puts junit:junit on the test classpath.
2. JUnit 5.8.1 → 6.1.3, with junit-platform-launcher on the test runtime and the two extension API changes JUnit 6 needs.
3. BackendTestExtension on JUnit's store cleanup, so it and the fixtures of [Subtask] Add a JUnit fixture framework for test containers and move every caller to it #14002 close resources the same way.

Testcontainers 1.21.4 → 2.0.5 is #13958 and lands between 2 and 3; 2.x drops junit:junit, which is why 1 comes first.

Parent issue

#13900

Order and PRs

#14103 (JUnit 4 removal) → #14083 (JUnit 6) → #14084 (Testcontainers 2, closes #13958) → #14085 (BackendTestExtension, closes this issue).