[TASK] Convert log levels from string to integer

In PSR-3, severity levels for log() can be given as a string.
If a string is given, it gets converted to the corresponding
constant of LogLevel.

Resolves: #48881
Releases: 6.2
Change-Id: I8d10d10b947a20bac483f7d3775a2d2fd7be52f6
Reviewed-on: https://review.typo3.org/21815
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn