Provide an option to backup world data when server version changes

Hard requirements:
- [ ] handle `TYPE=VANILLA`
- [ ] use `level` in `server.properties` and/or `LEVEL` var to account for both config styles
- [ ] create backup as tar.gz in the `/data` directory named with a prefix the same as the level name

Nice to have requirements:
- [ ] Identify data version of current world data by inspecting level.dat [wiki](https://minecraft.wiki/w/Data_version)
- [ ] Identify data version of Minecraft software about to be started by inspecting its version.json [wiki](https://minecraft.wiki/w/Data_version)

Open questions
- [ ] keep only one backup and overwrite or increment a backup index?

Start by tracing the server startup logic that reads server.properties, LEVEL, and TYPE=VANILLA, then review the /data layout and the level.dat and version.json formats. Done means world data is backed up as a tar.gz in /data with a level-name prefix, with the backup retention policy decided and both configuration styles handled.

Written by the indexing model from the issue text.