[TASK] Reduce severity for set_no_cache() from core

Reduce the severity if an internal flag is set and the a backend user
is logged in.

Resolves: #43735
Related: #43506
Releases: 6.1, 6.0, 4.7, 4.5
Change-Id: Ifd78f67d38b87180ab60df05167974705ee90352
Reviewed-on: https://review.typo3.org/20198
Reviewed-by: Philipp Gampe
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert