Make EverestConfig immutable

[oyvindeide]

Came across this comment:

ert/src/everest/config/everest_config.py

Lines 84 to 87 in 875935e

	# Makes property.setter work
	# Based on https://github.com/pydantic/pydantic/issues/1577#issuecomment-790506164
	# We should use computed_property instead of this, when upgrading to pydantic 2.
	class BaseModelWithPropertySupport(BaseModel):

, which might not be relevant any more. We did some work on make the everest config immutable, and if it is not fully immutable, we should make it immutable and remove property support.