Fix FLINK-34007 LeaderElector bug in 1.18

FLINK-34007 revealed a bug in the k8s client v6.6.2 which we're using since Flink 1.18. This issue was fixed with FLINK-34007 for Flink 1.19 which required an update of the k8s client to v6.9.0.

This Jira issue is about finding a solution in Flink 1.18 for the very same problem FLINK-34007 covered. It's a dedicated Jira issue because we want to unblock the release of 1.19 by resolving FLINK-34007.

Just to summarize why the upgrade to v6.9.0 is desired: There's a bug in v6.6.2 which might prevent the leadership lost event being forwarded to the client (#5463). An initial proposal where the release call was handled in Flink's KubernetesLeaderElector didn't work due to the leadership lost event being triggered twice (see FLINK-34007 PR comment)