Flink: Exclude Vintage and clean up test deps

[subhramit]

Partially addresses #12937.

Exclude junit and org.junit.vintage from the connector.test.utils and test.utils test dependencies in all three Flink modules (v1.20, v2.0, v2.1), and remove the now-redundant explicit flink-test-utils-junit dependency and its libs.versions.toml aliases.

Keep junit:junit as an explicit runtime-only dependency for Flink tests and integration tests in those modules as a temporary workaround for Flink's own transitive test infrastructure.

Full removal of JUnit 4 from the Flink test runtime remains blocked upstream by FLINK-25325.
Although the Iceberg Flink tests in this branch already use JUnit 5 extensions (for example, TestIcebergSourceFailover uses MiniClusterExtension, not MiniClusterWithClientResource), Flink's transitive flink-test-utils-junit implementation still requires JUnit 4 classes at runtime.

Testing

Verified via repo search that no Iceberg Flink test class under flink/ references MiniClusterWithClientResource, org.junit.rules, @Rule, or ExternalResource. Also ran:

./gradlew :iceberg-flink:iceberg-flink-2.0:dependencies --configuration testRuntimeClasspath | grep -i junit

(see output below in Details)
to confirm junit-vintage-engine and org.junit.vintage no longer resolve. junit:junit:4.13.2 still resolves intentionally as an explicit runtime-only workaround.

Details

+--- org.junit.jupiter:junit-jupiter:5.14.4
|    +--- org.junit:junit-bom:5.14.4
|    |    +--- org.junit.jupiter:junit-jupiter:5.14.4 (c)
|    |    +--- org.junit.jupiter:junit-jupiter-api:5.14.4 (c)
|    |    +--- org.junit.jupiter:junit-jupiter-engine:5.14.4 (c)
|    |    +--- org.junit.jupiter:junit-jupiter-params:5.14.4 (c)
|    |    +--- org.junit.platform:junit-platform-engine:1.14.4 (c)
|    |    +--- org.junit.platform:junit-platform-launcher:1.14.4 (c)
|    |    \--- org.junit.platform:junit-platform-commons:1.14.4 (c)
|    +--- org.junit.jupiter:junit-jupiter-api:5.14.4
|    |    +--- org.junit:junit-bom:5.14.4 (*)
|    |    \--- org.junit.platform:junit-platform-commons:1.14.4
|    |         \--- org.junit:junit-bom:5.14.4 (*)
|    +--- org.junit.jupiter:junit-jupiter-params:5.14.4
|    |    +--- org.junit:junit-bom:5.14.4 (*)
|    |    \--- org.junit.jupiter:junit-jupiter-api:5.14.4 (*)
|    \--- org.junit.jupiter:junit-jupiter-engine:5.14.4
|         +--- org.junit:junit-bom:5.14.4 (*)
|         +--- org.junit.platform:junit-platform-engine:1.14.4
|         |    +--- org.junit:junit-bom:5.14.4 (*)
|         |    \--- org.junit.platform:junit-platform-commons:1.14.4 (*)
|         \--- org.junit.jupiter:junit-jupiter-api:5.14.4 (*)
+--- org.junit.jupiter:junit-jupiter-engine:5.14.4 (*)
+--- org.junit.platform:junit-platform-launcher:1.14.4
|    +--- org.junit:junit-bom:5.14.4 (*)
|    \--- org.junit.platform:junit-platform-engine:1.14.4 (*)
|    |    +--- org.apache.flink:flink-test-utils-junit:2.0.0
|    |    |    +--- org.junit.jupiter:junit-jupiter:5.11.4 -> 5.14.4 (*)
|    |    +--- org.junit.jupiter:junit-jupiter:5.11.4 -> 5.14.4 (*)
|    |    |    \--- org.junit.jupiter:junit-jupiter-api:5.6.2 -> 5.14.4 (*)
|    +--- org.apache.flink:flink-test-utils-junit:2.0.0 (*)
|    +--- org.junit.jupiter:junit-jupiter:5.11.4 -> 5.14.4 (*)
\--- junit:junit:4.13.2

Also ran some focused tests:

./gradlew -DflinkVersions=1.20 :iceberg-flink:iceberg-flink-1.20:test --tests org.apache.iceberg.flink.TestFlinkConfParser
./gradlew -DflinkVersions=2.0 :iceberg-flink:iceberg-flink-2.0:test --tests org.apache.iceberg.flink.TestFlinkConfParser
./gradlew -DflinkVersions=2.1 :iceberg-flink:iceberg-flink-2.1:test --tests org.apache.iceberg.flink.TestFlinkConfParser
./gradlew spotlessCheck

[uros-b]

Just one ordering nit

[uros-b]

junit-v4 is inserted between junit-pioneer (line 213) and junit-platform-launcher (line 214), but the junit-* library block is strictly alphabetically ordered (junit-jupiter → junit-jupiter-engine → junit-pioneer → junit-platform-launcher → junit-suite-api → junit-suite-engine). Since v > s, junit-v4 should be placed after junit-suite-engine. A one-line reorder in the TOML is all that is needed.

[subhramit]

Done, thanks!

[subhramit]

Also cc'ing @mxm @nastra @huaxingao