[CANCEL][VOTE] Release Apache Druid (incubating) 0.15.0 [RC1]

[Jihoon Son]

Hi all,

Voting on 0.15.0-incubating RC1 is closed because of
https://github.com/apache/incubator-druid/issues/7869.

Now the PR has merged, so I will build RC2 and start voting on it soon.

Jihoon