Generator Settings are not present in properties file by default

Generator settings (`level-type`, `level-seed`, `generate-structures`, and `generator-settings`) are not appearing by default in the `server.properties` file, although they will still work if you manually set them. Is this a regression, or was it intentional?

Expected: https://paste.md-5.net/ikipojegiz.properties (1.16.5)

Got: https://paste.md-5.net/ihalamajip.properties (1.17)