[FIX] web: fix ModelFieldSelectorPopover arrows

Since [1], the model field selector popover has some style in order to
give its pointing arrow a custom color.
But the style only takes into account the upside arrow.
It should also take into account the downside arrow as this popover does
have the ability to switch direction when it could clip outside of
the viewport.

**Before this commit**
![top before]
![bottom before]
![left before]

**After this commit**
![top after]
![bottom after]
![left after]

[1]: bd4862774264c5169aa2dcf4cd80908c2f4c71bb
[left before]: https://github.com/odoo/odoo/assets/1159815/68387876-8372-4f6a-8a53-1ce1132547a0
[left after]: https://github.com/odoo/odoo/assets/1159815/5c80b095-efdd-4927-86fb-887d0a18fc3e
[bottom before]: https://github.com/odoo/odoo/assets/1159815/2bc2b5b4-ec90-4e10-ba6e-173ebeba9815
[bottom after]: https://github.com/odoo/odoo/assets/1159815/284a4e3c-db41-4d3d-ba5e-e6522c641770
[top before]: https://github.com/odoo/odoo/assets/1159815/b43ad7a9-e02f-451b-9871-707e5abdd694
[top after]: https://github.com/odoo/odoo/assets/1159815/efce4370-bf27-4ea8-8d65-e8bb2e9d9909

closes odoo/odoo#124038

X-original-commit: 3d8c55ce62cd5f21c18c1a50820ac551e9d98855
Signed-off-by: Romeo Fragomeli (rfr) <rfr@odoo.com>
Signed-off-by: Bruno Boi (boi) <boi@odoo.com>

addons/web/static/src/core/model_field_selector/model_field_selector_popover.scss

@@ -1,5 +1,10 @@
-.o_popover_field_selector .popover-arrow::after {
-    border-bottom-color: map-get($theme-colors, 'primary');
+.o_popover_field_selector {
+    &.o-popover-top > .popover-arrow::after {
+        border-top-color: map-get($theme-colors, 'primary');
+    }
+    &.o-popover-bottom > .popover-arrow::after {
+        border-bottom-color: map-get($theme-colors, 'primary');
+    }
 }
 
 .o_model_field_selector_popover {