MINOR: Update Scala to 2.13.8 in gradle.properties (#12273)
MINOR: Update Scala to 2.13.8 in gradle.properties (#12273)

It was updated in dependencies.gradle but this one was probably left out.

Reviewers: Ismael Juma <0794a5c7e497910d682bf2c54c66cc30760c0221@juma.me.uk>, Andras Katona <41361962+akatona84@users.noreply.github.com>
apache/kafka,apache/kafka,apache/kafka,lindong28/kafka,guozhangwang/kafka,guozhangwang/kafka,apache/kafka,guozhangwang/kafka,lindong28/kafka,lindong28/kafka,guozhangwang/kafka,lindong28/kafka