Build: Use Java 21 to publish snapshot to Maven

[pan3793]

Use the highest supported JDK version to publish snapshot to Maven, to fix https://github.com/apache/iceberg/actions/runs/15812361048/job/44565434608.

Both Java and Scala are configured with -release:11, it means even use a higher version of JDK to build, the output jars still compatible with Java 11.

  tasks.withType(JavaCompile.class).configureEach {
    options.encoding = "UTF-8"
    options.release = 11
  }

    tasks.withType(ScalaCompile.class) {
      scalaCompileOptions.keepAliveMode.set(KeepAliveMode.DAEMON)
      // `options.release` doesn't seem to work for ScalaCompile :(
      sourceCompatibility = "11"
      targetCompatibility = "11"
      scalaCompileOptions.additionalParameters.add("-release:11")
    }

[pan3793]

cc @huaxingao @pvary @RussellSpitzer