[TASK] Replace TSFE->fePreview with aspect on TYPO3v10+

[cweiske]

Using $GLOBALS['TSFE']->fePreview has been deprecated in TYPO3v10 and removed in v11. It generates a deprecation warning:

TYPO3 Deprecation Notice: Property $TSFE->fePreview is not in use anymore as this information is now stored within the FrontendPreview aspect. in typo3/sysext/frontend/Classes/Controller/TypoScriptFrontendController.php line 4101

Relevant changelog entries:

• https://docs.typo3.org/c/typo3/cms-core/10.4/en-us/Changelog/10.0/Feature-88791-IntroducePreviewAspectInContext.html
• https://docs.typo3.org/c/typo3/cms-core/11.3/en-us/Changelog/11.0/Breaking-91473-DeprecatedFunctionalityRemoved.html

This patch uses the new context aspect if it exists, and falls back to TSFE->fePreview in old TYPO3 versions.

Resolves: https://github.com/FluidTYPO3/vhs/issues/1775

How to test

Preview a disabled page in the frontend