[TASK] Add "Abstract" prefix to abstract task class

All abstract classes should be start with an Abstract
prefix in the class name.
Adjust the abstract task class to this naming convention.

Resolves: #42803
Releases: 6.0

Change-Id: If0b62d4ff61d9aa7ce44f61a15457faeabeab2d7
Reviewed-on: http://review.typo3.org/16331
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel