[FIX] base: gantt rng option duration_unit

Before this commit, the duration_unit view attribute was not present in the gantt RNG
But the JS code in the framework and in Studio allow for modifying it

So, this commit makes it an optional view attribute

OPW 1946216

closes odoo/odoo#32133

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

odoo/addons/base/rng/gantt_view.rng

@@ -25,6 +25,8 @@
             <rng:optional><rng:attribute name="consolidation"/></rng:optional>
             <rng:optional><rng:attribute name="consolidation_max"/></rng:optional>
             <rng:optional><rng:attribute name="consolidation_exclude"/></rng:optional>
+            <!-- Shouldn't be forward ported to >= saas-12.2 -->
+            <rng:optional><rng:attribute name="duration_unit"/></rng:optional>
             <rng:empty/>
         </rng:element>
     </rng:define>