[Server] Remove `Organization.is_expired` from datamodel

[FirelightFlagboy]

Currently when an admin-sys want to expired an organization, it need to set is_expired in the datamodel to true but _expired_on is not updated.

We should instead only rely on _expired_on since we can achieve the same behavior as with is_expired.

This does not need to break the existing API has we can easily set _expired_on to now() when patching is_expired is true.
And similarly, we can return is_expired if the date is due.

[mmmarcos]

It looks that both fields are updated:

parsec-cloud/server/parsec/components/postgresql/organization_update.py

Lines 34 to 37 in 51fa463

	fields = []
	if with_is_expired:
	fields.append("is_expired = $is_expired")
	fields.append("_expired_on = (CASE WHEN $is_expired THEN NOW() ELSE NULL END)")

Have you spotted a bug or a missing update elsewhere? Otherwise this issue is more about refactoring that enhancement.

[FirelightFlagboy]

I did not spotted a bug, but haven't tested it either.

Anyway the code seems to rely on the flag is_expired instead of checking _on_expired.

Relying on the date would simplify the management side (bms)