Map postgres :restrict_violation to :foreign_key constraint

[ashutoshrishi]

In PostgreSQL 18, this error code is now used for ON DELETE RESTRICT constraint violation. The existing :foreign_key_violation error code is raised for ON DELETE NO ACTION.

The behaviour changed in postgres commit
postgres/postgres@086c84b

The change is backwards compatible as the error code was not previously raised even though it existed in the postgres documentation. It is now correctly raised for the ON DELETE RESTRICT constraint.

Resolves #712

The example provided in the ISSUE now correctly returns a changeset instead of raising:

{:error,
 #Ecto.Changeset<
   action: :delete,
   changes: %{},
   errors: [
     children: {"does not exist",
      [constraint: :foreign, constraint_name: "child_parent_id_fkey"]}
   ],
   data: #Foo.Parent<>,
   valid?: false,
   ...
 >}

[josevalim]

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