Drop macOS 12

[i2h3]

We use Qt 6.10 by now and it requires macOS 13 or newer. This is an upstream dependency constraint we cannot change and need to react to.

To Do

• Take raised requirements in update system into account (Dynamic legacy platform channel routing with macOS < 13 support client_updater_server#231)
• Update the download site to reflect the changed minimum version requirement (still states "macOS 12+") - on this occasion maybe specify this centrally as a project-wide build setting which is only inherited by targets which currently and redundantly define it
• Remove run time code switches which control code execution based on macOS release (they look like if #available(…)).
• Update supported platform definitions of Swift packages (NextcloudFileProviderKit, NextcloudCapabilitiesKit, MacCrafter, TransifexStringCatalogSanitizer)
• Update Xcode project and its minimum deployment targets
• Update wherever you find the CMAKE_OSX_DEPLOYMENT_TARGET build setting for Xcode which includes CMakeLists.txt

[Rello]

updater server needs to handle this also

nextcloud/client_updater_server#231