Update macos runners (parallels 15, macos 10.14.6, xcode 11)

Depends on:

• cerbero!292 (merged)
• cerbero!291 (merged)
• cerbero!293 (merged)

Checklist:

• gst-mac-ci1:
  • gitlab-runner stop
  • Parallels 15
  • VM update
    • take snapshot
    • macOS 10.14.6
    • Xcode 11
      • first run completed
    • update cerbero source cache
  • gitlab-runner start
  • smoke test with a CI run
• gst-mac-ci2:
  • gitlab-runner stop
  • Parallels 15
  • VM update
    • take snapshot
    • macOS 10.14.6
    • Xcode 11
      • first run completed
    • update cerbero source cache
  • gitlab-runner start
  • smoke test with a CI run