[TASK] Re-work/simplify copyright header in JS files

This patch replaces the copyright/license header in JS files with a
new, simplified one. The new header does not show the year figure, nor
an author name, and refers to the LICENSE.txt file for the full
copyright information. License is: GPL2 or any later version.

Resolves: #59916
Releases: 6.3, 6.2
Change-Id: I312bd9682fc1cf30ec327ba5d17567a3e9609c7d
Reviewed-on: https://review.typo3.org/31164
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
(cherry picked from commit 1ae7b94b86a64ab6a86d7417a5dfb1f19777ee75)
Reviewed-on: https://review.typo3.org/31248