build: move back to 999-SNAPSHOT

[timefold-release]

At this point, the release of Timefold Solver Community Edition is ready to be published.
Artifacts have been uploaded to a staging repository.
Release branch has been created.

To finish the release of Timefold Solver,
please follow the steps below in the given order:

1. Release the staging repository on OSSRH.
2. Wait for the artifacts to reach Maven Central.
3. Undraft the release on Github. This will cause the release tag to be published, enabling the downstream release scripts.
4. Merge this PR. (Only do this after undrafting the release, otherwise the tag will point to the final SNAPSHOT commit.)
5. Delete the branch that this PR is based on. (Typically a button appears on this page once the PR is merged.)
6. Start and complete release automation for Timefold Solver Enterprise Edition.
7. Start and complete release automation
   for Timefold Quickstarts.
8. After finishing all above release automation, run
   the website release automation.
   9.[ ] Bump Solver version on start.spring.io by submitting a PR
   to this file.

Note: If this is a dry run,
none of the above applies and this PR should not be merged.