[TASK] Update readme of EXT:lowlevel

The lowlevel CLI tasks have been migrated to symfony commands and
therefore the readme has to be updated as well.

Resolves: #79996
Releases: master
Change-Id: If584569680bfcf769056b39b5472571deeb21807
Reviewed-on: https://review.typo3.org/51839


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Josef Glatz <josef.glatz@typo3.org>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>