[BUGFIX] Use correct HTTP method in GeneralUtility::getUrl

When fetching only headers from an URL there is no need
to download the whole content.
This was overlooked when migrating the core to use Guzzle.

Resolves: #85491
Releases: master, 8.7
Change-Id: I6475b405d51135372e022b5e133368f15a917687
Reviewed-on: https://review.typo3.org/57479


Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jürgen Venne <venne@schaffrath-digital.de>
Tested-by: Jürgen Venne <venne@schaffrath-digital.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>