Update git submodules
- Update extensions/PdfHandler from branch 'REL1_39' to d12466860c97f68d905e12eb2d47b2ac42680df7
- build: Update MediaWiki requirement to 1.39.0
Change-Id: Ic04c31fdea3a6140c9a024c19bd7528949c14c5a
- build: Update MediaWiki requirement to 1.39.0