[6.x] Add custom deleted_at column name for assertSoftDeleted

[dwightwatson]

This fixes issue #30110 where it isn't possible to use assertSoftDeleted if you're using a column name other than deleted_at. This allows the custom column name to be passed into the assertion, and it also adds support if you pass in a model instance.

A few things to note:

• I updated the model check to also include that it uses the SoftDeletes trait. It wasn't required previously but now that we call getDeletedAtColumn() we want to be sure that the method will be there. This would be backwards incompatible if you were using this assertion but not using the SoftDeletes trait, but I don't think that would be expected behaviour.

• I've popped $deletedAtColumn on as the last argument to the method so as not to break anything. However I have a gut feeling it should rather be between the $data and $connection arguments. Curious as to your thoughts in regard to this.

[dwightwatson]

That failure on 7.4 doesn't appear to be relevant, hopefully a rebuild will clear it up.

[driesvints]

@dwightwatson rebuild it for your