Drop support for Zotero 7, require Zotero 8+

[EwoutH]

Now that Zotero 8 is released, it might make sense to drop support for Zotero 7 and require Zotero 8 or higher. That will remove a lot of code path and complexities, and allows to use many of the new Zotero 8 features (and optimize against them).

Users on Zotero 7 of course then can keep using existing ZotSeek versions (1.6 and below).

See also: https://www.zotero.org/support/dev/zotero_8_for_developers