Update git submodules
- Update extensions/MultimediaViewer from branch 'REL1_39' to c311f4e878a759f026285a6521a345a93e2aaf15
- Merge "build: Update MediaWiki requirement to 1.39.0" into REL1_39
- build: Update MediaWiki requirement to 1.39.0
Change-Id: I9d08be742865ef4e6552eff8af3c72f431b383e9