Exclude JUnit4 dependency from classpath

### Feature Request / Improvement

We should apply the diff
```
diff --git a/build.gradle b/build.gradle
index cd79e45a7f..ed7133cc7e 100644
--- a/build.gradle
+++ b/build.gradle
@@ -182,6 +182,7 @@ subprojects {
exclude group: 'com.sun.jersey'
exclude group: 'com.sun.jersey.contribs'
exclude group: 'org.pentaho', module: 'pentaho-aggdesigner-algorithm'
+ exclude group: 'junit'
}
```

to completely exclude JUnit4 from the classpath.

There are unfortunately still a few other projects that depend on JUnit4
* Testcontainers depends on JUnit4 when using `GenericContainer`. As indicated by https://github.com/testcontainers/testcontainers-java/issues/970 this should be fixed in Testcontainers 2.0
* Flink currently also depends on JUnit4 as indicated in https://github.com/apache/iceberg/issues/12937 (Flink's `InternalMiniClusterExtension` still depends on having JUnit4 on the classpath)

### Query engine

None

### Willingness to contribute

- [ ] I can contribute this improvement/feature independently
- [ ] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time

Start in build.gradle at the subprojects dependency exclusions and review the linked Testcontainers and Flink constraints. Apply the requested JUnit4 exclusion and verify that dependency resolution no longer places JUnit4 on the classpath without breaking the affected projects.

Written by the indexing model from the issue text.