[BUGFIX] Change header tag in PaletteAndSingleContainer and adjust tab.html

This commit fixes a minor issue where the header
tag in PaletteAndSingleContainer was incorrectly
set as h4. The tag has been changed to h3 for
better readability and semantics. Simultaneously,
visually hidden headers were also added to tabs to
improve accessibility.

Resolves: #104021
Releases: main, 12.4
Change-Id: Ic2301eabb23d1d17169056ec8e9ec40a5824c52c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84822


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Oliver Bartsch <bo@cedev.de>