[improve][build] Set --enable-native-access=ALL-UNNAMED for test JVMs on Java 24+

[lhotari]

Motivation

Netty loads its native libraries (epoll, io_uring, tcnative) through
java.lang.System::loadLibrary, which became a restricted method in Java 24. Any test JVM that
touches a Netty native transport therefore prints to stderr:

WARNING: A restricted method in java.lang.System has been called
WARNING: java.lang.System::loadLibrary has been called by io.netty.util.internal.NativeLibraryUtil
         in an unnamed module
WARNING: Use --enable-native-access=ALL-UNNAMED to avoid a warning for callers in this module
WARNING: Restricted methods will be blocked in a future release unless native access is enabled

Pulsar already runs CI on JDK 25, so this is noise on every affected test JVM. It also
interferes with tests that assert a process wrote nothing to stderr, and, as the warning says,
restricted methods will eventually be blocked rather than warned about.

bin/pulsar already passes --enable-native-access=ALL-UNNAMED for the server, and #26365 adds
it to the CLI scripts. This PR does the same for the test JVMs.

Modifications

In build-logic/conventions/src/main/kotlin/pulsar.java-conventions.gradle.kts:

// Effective Java major version used to run tests: the -PtestJavaVersion override when set,
// otherwise the JVM running Gradle.
val testJavaMajorVersion = testJavaVersion.orNull ?: JavaVersion.current().majorVersion.toInt()

and, inside tasks.withType<Test>().configureEach:

if (testJavaMajorVersion >= 24) {
    jvmArgs("--enable-native-access=ALL-UNNAMED")
}

The version gate matters because the option is not accepted by older JVMs, and tests can be run
on an older JDK via -PtestJavaVersion. The effective version therefore honours that toolchain
override when present and falls back to the JVM running Gradle, so the gate is correct in both
cases.

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Verified locally that the conditional actually takes effect, in both directions, by inspecting
the test worker command line with --rerun-tasks -i:

• with the threshold temporarily lowered to 21 on a Java 21 JVM, --enable-native-access=ALL-UNNAMED
  appears in the spawned test JVM arguments;
• with the threshold at 24 on the same Java 21 JVM, it does not appear.

Also ran ./gradlew :buildtools:compileJava to confirm the convention plugin still compiles, and
a targeted test to confirm test execution is unaffected.

Does this pull request potentially affect one of the following parts:

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

[void-ptr974]

LGTM