Prerelease 21.0.0-RC1

This is the first release-candidate for the 21.x.x series.

More detailed release notes are still to be written, use at your own peril.

Relevant changes compared to 20.3.1:

• #143: Dependency updates: We now depend on spring-boot 2.5, Flyway 7, and (optionally) tokens 0.14.0.
  This might mean that this doesn't work with Java 8 anymore (feedback appreciated).
  This is using the new Flyway callback infrastructure. If you are using this, please be careful.

• #141: Possibility to persist multiple events in bulk (using just request to the DB).
  This should improve the performance if you create many events in the same transaction.
  (The API for this feature is not yet finalized, please give feedback on this, specifically
  the way data change events are submitted. Comments in #141, please.)

Note that this Prerelease was built from a merge of the two feature branches, before they were merged to master.
We might do incompatible changes between this and the final release of 21.0.0 – only use this if you are prepared to work with this.