CI: MacOS front-ends

[ajtribick]

Currently we are building SDL, GTK2, Qt5 and Qt6 front-ends for MacOS in the CI. Do we need all of these - in particular, can we remove GTK2 and Qt5?

[Levin Li]

i'm ok with dropping GTK2 and QT5. i don't think anyone would be using GTK2 on Mac building Celestia, and QT6 is homebrew's default for installing qt now.

[Hleb Valoshka]

Let's go.