[FIX] website: remove header font color transitions

Before this commit, the font color was subject to animations, this
commit removes that because it doesn't look good with a header over the
content.

Steps to reproduce the bug:
- Edit a website
- Set the header to "over the content"
- Put a dark background color on this header
- Scroll down and then up

=> The text color flicker.

Part-of: odoo/odoo#99732

addons/website/static/src/scss/website.scss

@@ -995,7 +995,7 @@ header {
     }
     &:not(.o_header_no_transition) {
         #top_menu_container {
-            transition: all $-transition-duration;
+            transition: all $-transition-duration, color 0ms;
         }
         .navbar-brand {
             transition: margin $-transition-duration, font-size $-transition-duration, opacity $-transition-duration ease-out;