ci(circleci): prevent timeout on circle-ci macos node10 build (#7651)

The circle ci job for macOS on node 10 currently fails because it takes too long
(both PR's #7649 and #7650 don't green because of that). A big chunk of the time
is taken by updating homebrew - which in this macOS (the one connected to xcode 9
=> macOS 10.12) does not contain node 10 by default.

This change:
- sets the required xcode version to 11.0 -> this gets us a more up to date macOS
  version (except for node 6, which isn't available in the homebrew associated
  with xcode 11)
- removes the brew update from the macOS node 10 job