gradle: replace deprecated classifier property

The `classifier` property of `Jar` has been deprecated few major
versions back, and no longer supported in the newest Gradle 8.0. Replace
`classifier` with `archiveClassifier` as suggested in docs:
https://docs.gradle.org/7.6/dsl/org.gradle.api.tasks.bundling.Jar.html#org.gradle.api.tasks.bundling.Jar:classifier

Fixes #6825.

Signed-off-by: Ruoyu Zhong <zhongruoyu@outlook.com>

build.gradle

@@ -124,7 +124,7 @@ allprojects {
   version = rootProject.version
 
   task sourcesJar(type: Jar, dependsOn: classes) {
-    classifier = 'sources'
+    archiveClassifier = 'sources'
     from sourceSets.main.allSource
     jar.reproducibleFileOrder = true
     jar.preserveFileTimestamps = false
@@ -976,4 +976,3 @@ task cloudsmithUpload {
     }
   }
 }
-