[FIX] portal: use proper muted color for portal chatter published dates

The color that was used for portal chatter published dates was the muted
color that we use at some places in the backend. The portal screens
being frontend screens, it of course did not work with all website color
schemes.

This simply uses the `$text-muted` color to fix the issue. This probably
needs refactoring in master to avoid those extra CSS rules that could
simply be gone with proper bootstrap XML structures.

opw-3146164

closes odoo/odoo#116968

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>