[TASK] Use default GPLv2 LICENSE.txt file

Our LICENSE.txt files contain a custom header
mentioning details of a 3rd party library.

The patch resets LICENSE.txt files to the
default GPLv2 version [1] and moves the 3rd party
credits to the backend about module, which
already lists other credits. This obeys the
authors whish to set a link to his work.

The default LICENSE.txt files have the additional
advantage that services like github should
recognize the project as GPLv2 licensed project.

[1] https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt

Resolves: #96837
Releases: main, 11.5, 10.4
Change-Id: I63a4cd286ea3714667688d12347034b9ae67f276
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73432


Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>