[TASK] Remove last usage of t3x

The label in the extension manager must be adopted as t3x is not allowed
anymore since #92590.
Furthermore, the upload allows only the selection of zip files to make it
easier for users.

Resolves: #98438
Related: #92590
Releases: main, 11.5
Change-Id: I7e2a616741f3c0355f09fb83cf85763f004bb55a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75870


Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>