Modal primary button should be the one in the corner (left if buttons aligned
left, right if buttons aligned right) as in the Bootstrap doc.
More than a simple guideline, the user will intuitively click on the button in
the corner as it is supposed to be the primary one.

Plus some DOM fixes (o_technical_modal was missing, H2 was too big especially
since BS4)

Closes #29890
task-1922120