[DOC] Add backwards-compatible example for getRequest()

Because the API has been introduced with Fluid v2, but hasn't been
used by TYPO3 < 13.3, a few extra checks are necessary.

Resolves: #104777
Releases: main
Change-Id: If4bb29dc9b6b5b0e87ec73307a2a0bca285c4318
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85824


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Simon Praetorius <simon@praetorius.me>
Tested-by: Simon Praetorius <simon@praetorius.me>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>