modrinth: support projects that declare "unknown" as server-type

Until https://github.com/modrinth/code/issues/2590 is resolved, there are projects like https://api.modrinth.com/v2/project/dynmap that report a `server_side` of "unknown" even though that is not a documented enum value in the project model: https://docs.modrinth.com/#tag/misc/operation/statistics

[Reported in Discord](https://discord.com/channels/660567679458869252/660569755320844308/1255637027228090398)

```
[mc-image-helper] 21:31:57.397 ERROR : 'modrinth' command failed. Version is 1.37.3
me.itzg.helpers.errors.GenericException: Failed to parse response body into list of class me.itzg.helpers.modrinth.model.Project: Cannot deserialize value of type `me.itzg.helpers.modrinth.model.ServerSide` from String "unknown": not one of the values accepted for Enum class: [optional, unsupported, required]
```

Start with the Modrinth command's Project response parsing and the ServerSide model named in the error. Reproduce the failure using the dynmap project response, then verify that a server_side value of "unknown" no longer prevents the project list from being parsed successfully.

Written by the indexing model from the issue text.