[FIX] account: display amount due field in boxed layout

Versions:
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- 14.0+

Steps to reproduce:
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1. Settings -> Configure document layout
2. Select the Boxed layout
3. Go to Invoices -> create a new invoice and register payment
4. Click on Preview
5. The field “Amount due” is in the wrong color and barely visible

Issue:
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The field “Amount due” is in the wrong color and barely visible in
invoice boxed layout

Cause:
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The issue is happening because we have `<strong>`  before the text
”Amount due” and it changes the font color to gray because of the CSS definitions

Solution:
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We need to make the text bold in a different way, to do it we can use the bootstrap
class font-weight-bold, this way we keep it bold and do not break the colors

OPW-3374092

closes odoo/odoo#130457

X-original-commit: 0c4badb0323414eb267be877109829ac8071dabe
Signed-off-by: Brice Bartoletti (bib) <bib@odoo.com>
Signed-off-by: Matheus Leal Viana (malv) <malv@odoo.com>