Do not allow modifying default taxonomy fields

[allisonking]

Is your feature request related to a specific problem?

https://github.com/ethyca/fides/pull/976 adds a column is_default to the taxonomy tables. If an entity is default, we should prevent users from being able to edit them. For example, we don't want a user to be able to change a default taxonomy entity to not a default one.

Describe the solution you'd like

In crud.py's update, upsert, and delete, first check if the entity is one of the taxonomy fields, and if so, check if it is a default field. If it is, do not allow modifying it.

Describe alternatives you've considered, if any

A description of any alternative solutions or features you've considered.

Additional context

Do we also need to prevent users from adding a default taxonomy entity in the first place?