Update git submodules

* Update platform/tools/dokka-devsite-plugin from branch 'master'
  to a6b702f55e587be21cb5dc4be17dd43bd4fb2ce5
  - Update upstream dokka to 1.6.20-M1-dev148
    
    This contains one regression: go/dokka-upstream-bug/2388
    This contains one unrelated improvement to logging.
    This contains no changes to integration test goldens.
    
    Change-Id: I98d912c3d186ea8281a1bcd249ce56782ce51b43