Update macos runners (macos 10.15, xcode 11.2.1, gitlab-runner 12.5.0)

Depends:

• cerbero!325 (merged)
• Soft dep on cerbero!323 (merged)
• !185 (merged)
• !186 (merged)

Checklist:

• gst-mac-ci1:
  • gitlab-runner stop
  • update gitlab-runner 12.5.0
  • parallels update
  • VM update
    • take snapshot
    • macOS 10.15
    • Xcode 11.2.1
      • first run completed
    • move cerbero host cache to ~/cerbero
    • update cerbero source cache
  • register catalina runner with tags `gst-macos-10.15,gst-ios-13.2
  • gitlab-runner start
  • smoke test with a CI run
• gst-mac-ci2:
  • gitlab-runner stop
  • update gitlab-runner 12.5.0
  • parallels update
  • VM update
    • take snapshot
    • macOS 10.15
    • Xcode 11.2.1
      • first run completed
    • move cerbero host cache to ~/cerbero
    • update cerbero source cache
  • register catalina runner with tags `gst-macos-10.15,gst-ios-13.2
  • gitlab-runner start
  • smoke test with a CI run