message: Update release branch script as main branch is now "main".
  See
diff: @@ -7,7 +7,7 @@ import sys
  # The branch being merged from.
  # This is the one that contains day-to-day development work.
  -MASTER_BRANCH = 'master'
  +MASTER_BRANCH = 'main'
  # The branch being merged into.
  # This is the release branch that users reference.
  LATEST_RELEASE_BRANCH = 'v1'
  