[BUGFIX] ExtJS / normalize table > th

Browsers center text in <th> but the header text should
be aligned to the left. This issue arose when ExtJS
normalize reset was removed. Simple explicit definition.

Resolves: #57211
Releases: 6.2
Change-Id: Ib235d9e9e2beabac01a62b10cedea08faacbd896
Reviewed-on: https://review.typo3.org/28679
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters