Use Pleroma-compatible version string

[mkljczk]

Summary

As documented here: https://gitlab.com/soapbox-pub/soapbox/-/blob/develop/docs/development/developing-backend.md

This is implemented by other non-Mastodon backends utilizing Mastodon API, including Pixelfed, Pleroma, Mitra, Friendica…

[artonge]

Makes sense since we recently moved to a mastodon like API, but I don't think that we have implemented the endpoints for instance, apps, and oauth. Instance should be moderately easy to implement, but apps and oauth, not sure if we will support that. Maybe we can use the Nextcloud server endpoints for that ? Accounts and statuses should be in a correct state as they are use by the front end.