Commit [1] altered the way the FieldMany2Many behaves with respect
to 'create' and 'delete' options. Indeed, for many2many fields,
adding or removing records doesn't mean "creating" or "deleting"
records, as it is only about adding/removing records to/from a
relation. This is completely fine and correct.

Unfortunately, a feature has been lost in the process: it is no
longer possible to state that a many2many field should be editable
but should not allow to add (or remove) record to the relation.

This commit fixes the issue by adding two new options: 'link' and
'unlink' for that purpose.

[1] https://github.com/odoo/odoo/commit/c98579d25af01c14df4baf57fb4652f3e7469096



opw~2466213

closes odoo/odoo#67358

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>