Prerelease 21.1.0-RC4

This is a prerelease used for internal testing.

Compared to 21.0.0, this contains changes from these PRs:

• #169 – spring-core dependency bump
• #166 – documentation of spring properties
• #171 – batch-delete eventlog entries, implementing #159
• #173 – add QueryStatementBatcher
• #167° – replace SimpleSnapshotEventProducer by static of-Methods
• #176° – Bugfix: no stups token bean if not needed
• #179° – use Zonky embedded postgres instead of opentable one for tests
• #177° – Add DELETE permission to the eventlog table (fixing #101)
• #175° – introduce "switch off all submission" toggle
• #174° – Delete after insert

° – Some of these were not yet merged when this release was created, for those we have the latest state as-of 2023-08-18.