Add release candidates to release procedure

Dask [2.26.0](https://docs.dask.org/en/latest/changelog.html#id1) (side note: anchor tags change with each release, so this link will be incorrect soon) included https://github.com/dask/dask/pull/6393, a major change to how casting and delegation works in Dask.

Unfortunately this change was incomplete and testing did not catch things like https://github.com/dask/dask/issues/6611 and https://github.com/dask/dask/issues/6631.

Has the Dask community considered adding a step to the [release procedure](https://github.com/dask/dask/blob/master/docs/release-procedure.md) in which a [release candidate](https://en.wikipedia.org/wiki/Software_release_life_cycle#Release_candidate) is published and possibly voted upon before becoming an official release?

This discussion is related but separate from the semantic versioning discussion happening in https://github.com/dask/community/issues/93.