[IMP] mail: keep old subtype subscription on change on parent record

With this commit, when changing subtype subscription from a parent
record (e.g. project of task), then it does not update subtype
subscription of existing records. Changes on parent record should
only affect new record afterwards.

Task-Id 1974055

Closes #34150

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>