Replace deprecated Jar.classifier with archiveClassifier

The Jar task property `classifier` was deprecated in Gradle 5.1 and
removed in Gradle 7.0. Builds using Gradle 7+ fail with:

  Could not set unknown property 'classifier' for task ':idl-parser:sourceJar'
  of type org.gradle.api.tasks.bundling.Jar.

Replace both usages with the current `archiveClassifier` property,
which has been available since Gradle 5.1 and is the supported API
going forward.

This unblocks projects that depend on IDL-Parser (e.g. Fast-DDS-Gen)
from upgrading their Gradle wrapper to 7.x or later.

Signed-off-by: Tuguberk <akbulut.tugberk@gmail.com>

Author: Tuguberk

build.gradle

@@ -71,7 +71,7 @@ jar {
 }
 
 task sourceJar(type: Jar, dependsOn: classes) {
-   classifier = 'sources'
+   archiveClassifier = 'sources'
    from sourceSets.main.allJava
 }
 
@@ -91,7 +91,7 @@ publishing {
          from components.java
 
          artifact sourceJar {
-            classifier 'sources'
+            archiveClassifier = 'sources'
          }
       }
    }