Added "KeepUniqueBackups" to config.yml + 1.18.X support*.
With this feature you can prevent unique backups from being deleted.

So, for example you have backed up a world 30 days ago but did not made a new backup and you configured that backups which are older than 30 days will be deleted automatically, this world folder will not be deleted at the next automatic backup deletion, because it is the only backup of this world.

*1.18.X support: There should be no problems in running this plugin on the latest version of spigot. Please let me know if you find any bugs / errors! This plugin uses only minimal spigot api related code, that is why there should be no problems in running this plugin on coming minecraft / spigot versions.