this commit removes the usage of .form-group class which is deprecated
since BS5.

Here is the css rules that was used:

a) https://github.com/twbs/bootstrap/blob/8fa0d3010112dca5dd6dd501173415856001ba8b/dist/css/bootstrap.css#L1997
As we can see, it simply adds a `margin-bottom` of `1rem` which
corresponds to the `.mb-3` BS class.

b) https://github.com/twbs/bootstrap/blob/8fa0d3010112dca5dd6dd501173415856001ba8b/dist/css/bootstrap.css#L2326
As we already checked all `form-inline` in [1] and [2], we don't have to
do anything about these rules.

'''Breaking change: Dropped form-specific layout classes for our grid
system.
Use our grid and utilities instead of .form-group, .form-row, or
.form-inline.'''

https://getbootstrap.com/docs/5.0/migration/#forms

Notes:
- `position: relative` is already on `#new-password-group`.
- `.field-db`, `#editor-media-image`, `.unsplash_img_container` and
`#url-form-group` seems unused.
- Sometimes margins are unnecessary because of blocks overlapping.
  (e.g. `margin-bottom` is not needed if margin-top is set on the
  following node)
- CSS rules applied on `.s_website_form_rows > .form-group` are now in
the XML by adding `mb-0 py-2` BS classes.

Follow-up of:
[1] https://github.com/odoo/odoo/pull/97967
[2] https://github.com/odoo/enterprise/pull/30343



closes odoo/odoo#100052

Related: odoo/enterprise#31261
Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>