Delete unused objects

[FelixMalfait]

Fixes #7113

[Weiko]

Great @FelixMalfait ! I think sync-metadata won't work there because we are removing multiple objects that depend on each others and we don't have a clear control on the order of deletion.
Also we might need a command for people that created custom relations between their objects and those deleted tables 🤔. We could do like we do with deprecated fields and instead of deleting those objects (and tables), we convert them to custom objects, that would be the "easy" solution but definitely not the best there I think
Edit: we could also update the sync-metatadata logic so it properly removes the foreign key constraints before deleting objects

[Weiko]

I've updated your PR.

• Object deletion migration now contains DROP_FOREIGN_KEY mutations for relations, this will allow us to delete objects which are dependencies from other tables
• Field metadata deletion mutations now delete the associated indexes, this is because IndexFieldMetadata are already deleted from CASCADE rule but because our indexes always have deletedAt as default column, the parent IndexMetadata remains and breaks the sync because it does not match the standard definition