Update VE core submodule to master (6e68242)

New changes:
c06fa64 Account for isValid() becoming async
12e4840 Account for WhitespacePreservingTextInputWidget#getValue being called early

Local changes:
* Make MWLinkTargetInputWidget#isValid asynchronous

Bug: 71237
Bug: 71246
Change-Id: Iab83e743f99973f01a54b23fd5ddf1081f7effd6