[FLINK-16163][build] Migrate to flink-shaded-zookeeper

[zentol]

Based on #11131.

With this PR the project now uses the zookeeper 3.4 version provided by flink-shaded. This dependency also bundles curator 4, rendering flink-shaded-curator obsolete and implicitly addressing FLINK-14177.

The vast majority of changes in this PR are imports.
The only notable .java file is ZooKeeperLeaderElectionTest, which adjusts some mocking behavior. With curator 4 the mocking resulted in errors, but I admittedly have not looked into it too much.

Beyond that, there is now a dependency management entry in the root pom for flink-shaded-zookeeper, the flink-runtime dependencies on zk/curator were replaced with the flink-shaded dependency. flink-runtime no longer bundles zookeeper; this responsibility is now deferred to flink-dist.

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[tillrohrmann]

Thanks for replacing ZooKeeper with our shaded ZooKeeper @zentol. LGTM. +1 for merging.