[TASK] Make use of .t3-table in EXT: scheduler

Use <table class="t3-table"> in backend module.

Change-Id: I3a6e899a4980b48beb47c46e6275ad591be332ad
Resolves: #52564
Related: #49592
Releases: 6.2
Reviewed-on: https://review.typo3.org/24433
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn