[BUGFIX] Streamline CSS after Bootstrap 5 update

Some very old code is removed in CSS definitions back from TYPO3 v7 (where
we had <img> tags).

In addition, some more Bootstrap 5 adaptions are made from the
initial update to increase contrast.

Resolves: #93179
Releases: master
Change-Id: If303c5a891e3b833f448fce65259c2751a4f7124
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68952


Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>