KAFKA-16589: Consider removing ClusterInstance#createAdminClient since callers are not sure whether they need to call close

[FrankYang0529]

Sometimes we close the admin created by createAdminClient, and sometimes we don't. That is not a true problem since the ClusterInstance will call close when stopping.

However, that cause a lot of inconsistent code, and in fact it does not save much time since creating a Admin is not a hard work. We can get bootstrapServers and bootstrapControllers from ClusterInstance easily.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[chia7712]

We are doing the migration so can this PR get pending before we see more use cases?