Geo: Keep the minimum cursor last event

What does this MR do?

Fixes this condition:

In which the PruneService prunes up-to the last processed ID (inclusive).

• Last event ID seen from primary needs at least one event in the DB to kept
• Last event ID processed by cursor needs the min cursor last event to be kept

This bug also would block finding the event processing lag time for https://gitlab.com/gitlab-org/gitlab-ee/issues/5386. Also similar for https://gitlab.com/gitlab-org/gitlab-ee/issues/5969.

After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer