[BUGFIX] Fix appearance of FlexForm tab content

FlexForm was using the old Bootstrap "panel" classes for its sheet tab
content. Since the Bootstrap 5 update, the underlying "box-shadow" CSS
property became visible. This is now fixed by simply using the
"form-section" class like everywhere else for FormEngine container.

In addition, the negative margin hack for this specific case can now be
removed.

Note: The Tabs.html template is still used in the deprecated
ModuleTemplate->getDynamicTabMenu() method. As the markup has changed,
it could look a bit different compared to TYPO3 v11, in case custom CSS
was applied, which is very unlikely.

Resolves: #99816
Releases: main
Change-Id: I74b67fba421569669cbe3bf073097f6a18006acb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77691


Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>