Replace deprecated Jar.classifier with archiveClassifier

[Tuguberk]

Problem

Building IDL-Parser with Gradle 7.0 or later fails during configuration:

A problem occurred evaluating project ':idl-parser'.
> Could not set unknown property 'classifier' for task ':idl-parser:sourceJar'
  of type org.gradle.api.tasks.bundling.Jar.

This blocks consumers of IDL-Parser (notably Fast-DDS-Gen) from upgrading their Gradle wrapper beyond 6.x, which in turn prevents building on systems running Java 11+ as the primary JDK.

Root cause

The classifier property on Jar tasks was deprecated in Gradle 5.1 and removed in Gradle 7.0. Two usages remain in build.gradle:

// line 74 — task declaration
task sourceJar(type: Jar, dependsOn: classes) {
    classifier = 'sources'   // ← removed in Gradle 7
    ...
}

// line 94 — artifact block inside publishing
artifact sourceJar {
    classifier 'sources'     // ← removed in Gradle 7
}

Fix

Replace both with archiveClassifier, the supported API since Gradle 5.1:

task sourceJar(type: Jar, dependsOn: classes) {
    archiveClassifier = 'sources'
    ...
}

artifact sourceJar {
    archiveClassifier = 'sources'
}

Impact

• No behaviour change — the produced artifact still carries the sources classifier.
• Unblocks Gradle wrapper upgrades to 7.x / 8.x in dependent projects.
• Backwards compatible: archiveClassifier is available from Gradle 5.1 onward.

Testing

Verified that IDL-Parser builds cleanly under Gradle 8.5 with Java 21 after this change, as part of a full Fast-DDS-Gen build on macOS arm64.