[SECURITY] Raise Fluid Standalone dependency

Raise Fluid Standalone dependency to the next stable version
which fixes an important XSS issue when escaping
ternary operators.

Used composer command:
  composer req typo3fluid/fluid:^2.6.1 --prefer-lowest

Resolves: #88288
Releases: master, 9.5, 8.7
Security-Bulletin: TYPO3-CORE-SA-2019-013
Change-Id: I04f32d8d01f893bc26ff21aa0c079c85e9db85b9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60693


Reviewed-by: Claus Due <claus@phpmind.net>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>