Bug 38379: Remove obsolete Bootstrap classes from installer templates

Bug 38379: Remove obsolete Bootstrap classes from installer templates

This patch makes some minor markup changes to the installer and
onboarding templates so that obsolete Bootstrap grid classes are
removed. The patch also adds another breakpoint to the grid classes to
that the content adjusts well at smaller viewport sizes.

To test, apply the patch and rebuild the staff interface CSS.

- Drop and recreate the Koha database. Run 'restart_all' if necessary.
- Navigate to the staff interface. You should be presented with the
  installer login. The form should be centered on the page.
- Log in and follow the installer and onboarding steps to confirm that
  the layout looks correct on each page.

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>