[FIX] link_tracker, pos_mercury: use unescape from html

As werkzeug.utils.unescape is deprecated in werkzeug 2.0 and the
markupsafe version of unescape just uses the built-in html.unescape [0],
we can use directly the same built-in method.

[0]  pallets/markupsafe@c35603a

Part-of: odoo/odoo#98081