CI: Update macOS deployment target to macOS 12

Qt 6.8.3 only supports macOS 12+. It does not support macOS 11, and OBS
Studio will fail to launch on macOS 11 as a result. We should reflect
that with our deployment target.

https://doc.qt.io/qt-6.8/supported-platforms.html

CMakePresets.json

@@ -33,7 +33,7 @@
       "generator": "Xcode",
       "binaryDir": "${sourceDir}/build_macos",
       "cacheVariables": {
-        "CMAKE_OSX_DEPLOYMENT_TARGET": {"type": "STRING", "value": "11.0"},
+        "CMAKE_OSX_DEPLOYMENT_TARGET": {"type": "STRING", "value": "12.0"},
         "OBS_CODESIGN_IDENTITY": {"type": "STRING", "value": "$penv{CODESIGN_IDENT}"},
         "OBS_CODESIGN_TEAM": {"type": "STRING", "value": "$penv{CODESIGN_TEAM}"},
         "OBS_PROVISIONING_PROFILE": {"type": "STRING", "value": "$penv{PROVISIONING_PROFILE}"},