Update git submodules
- Update extensions/Poem from branch 'REL1_39' to 153e3993b3b2c28b976fc78bf2bb59d5deadaf8e
- build: Update MediaWiki requirement to 1.39.0
Change-Id: I4b7bb62cd22a6d7a1d8f88d25d946f45ea8538e3
- build: Update MediaWiki requirement to 1.39.0