[TASK] Update the license headers

This change updates all PHP file license headers to the new
and shorter version used in TYPO3 CMS. It also adds a
license.txt.

Resolves: #59961
Change-Id: I0578c313a8ecbdee144523a806630bacf93d4965
Reviewed-on: https://review.typo3.org/31190
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe