[FIX] mrp: show uom correctly

Before this commit, field`product_uom_category_id` was related to `product_id` which is not required and will be visible when Variants are not enabled so on such conditions no `Unit of Measure` was displayed. (As product_uom_category_id is False )

With this commit, we make this field related to `product_tmpl_id` which sets correct `product_uom_category_id` which thus we can see correct list of  `Unit of Measure`.

closes odoo/odoo#48687

Signed-off-by: Rémy Voet <ryv-odoo@users.noreply.github.com>

addons/mrp/models/mrp_bom.py

@@ -46,7 +46,7 @@ class MrpBom(models.Model):
         'uom.uom', 'Unit of Measure',
         default=_get_default_product_uom_id, required=True,
         help="Unit of Measure (Unit of Measure) is the unit of measurement for the inventory control", domain="[('category_id', '=', product_uom_category_id)]")
-    product_uom_category_id = fields.Many2one(related='product_id.uom_id.category_id')
+    product_uom_category_id = fields.Many2one(related='product_tmpl_id.uom_id.category_id')
     sequence = fields.Integer('Sequence', help="Gives the sequence order when displaying a list of bills of material.")
     routing_id = fields.Many2one(
         'mrp.routing', 'Routing', check_company=True,