Since the merge of Bootstrap 5 at [1], a background-color and an opacity
are added on the `<hr>` elements. That way the <hr> now follows the
color of the surrounding text but are faded a bit. The BS4 behavior was
to use a lightgray (transparent black).
With the adaptation at [1], it was chosen to be compatible by forcing
that opacity to 1 and re-forcing the previous transparent black. This
commit chooses to instead keep the nice auto behavior of BS5, at worst
this will thus change lightgray <hr> to the color of the surrounding
text (faded a bit), it should not be a breaking change. The separator
snippet however keeps its full compatibility as we want to let users
choose the exact color they want so we cannot add a default opacity (we
may want to improve the UX later on).
[1]: https://github.com/odoo/odoo/commit/971e5a91aab96d36129a823e03f1f9f1b1293968
Part-of: odoo/odoo#98028
Benjamin Vray
570cbc194f