Apple Developer ID missing: Mixxx can’t be opened because it is from an unidentified developer

[jus]

The 2.1 release can´t be opened at least on macOS 10.11+ ( 10.13.x is current) with default system security settings.

The Mixxx application has no Apple Developer ID, it expired, was not renewed, and was removed lately. Can not find it atm in the git log.

Error message on 1st launch
``
“Mixxx” can’t be opened because it is from an unidentified developer
Your security preferences allow installation of only apps from the App Store and identified developers.
``

To start Mixxx, a user has to go trough some hassle:
* System Preferences -> Security&Privacy

  ``Mixxx`` was blocked from opening because it is not from an identified developer --> Open anyway

or (not recommended)
* open a terminal
  sudo spctl --master-disable

  System Preferences -> Security&Privacy -> Allow apps downloaded from anywhere

Makes a really bad impression, we should get a new DeveloperID

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/9248