Update copyright year to 2021

Resolves: #93769
Releases: master, 10.4, 9.5
Change-Id: Iec3214352aca9df579a3a1574eaf61222a6d2689
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68500Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>