[FIX] digest: add missing depends on is_subscribed field

Currently subscribed field is not computed again if user_ids field value
changes on a digest. This has probably few consequences in current code
but better be sure this field is correctly updated in a given transaction.

Task-2641394 (Digest emails sending improvement)
Task-2582128 (Digest onboarding and usage improvement)

X-original-commit: 3a7c1348
Part-of: odoo/odoo#79877