[tests] Use the correct OS version for Mac Catalyst. (#14143)

This fixes the CheckManagedFilters tests on Mac Catalyst on older macOS versions:

    [FAIL] CheckManagedFilters :   Managed filters not found for CIAreaMinMax, CIAreaMinMaxRed, CIAttributedTextImageGenerator, CIBarcodeGenerator, CIBicubicScaleTransform, CIBlendWithBlueMask, CIBlendWithRedMask, CIBokehBlur, CICameraCalibrationLensCorrection, CIColorCubesMixedWithMask, CIColorCurves, CICoreMLModelFilter, CIDepthBlurEffect, CIDepthToDisparity, CIDisparityToDepth, CIDither, CIDocumentEnhancer, CIEdgePreserveUpsampleFilter, CIGaborGradients, CIGuidedFilter, CIKeystoneCorrectionCombined, CIKeystoneCorrectionHorizontal, CIKeystoneCorrectionVertical, CIKMeans, CILabDeltaE, CIMeshGenerator, CIMix, CIMorphologyGradient, CIMorphologyMaximum, CIMorphologyMinimum, CIMorphologyRectangleMaximum, CIMorphologyRectangleMinimum, CIPaletteCentroid, CIPalettize, CIPerspectiveRotate, CIRoundedRectangleGenerator, CISaliencyMapFilter, CISampleNearest, CITextImageGenerator
      Expected: 0
      But was:  39

tests/common/PlatformInfo.cs

@@ -31,7 +31,7 @@ namespace Xamarin.Tests
 			string version;
 #if __MACCATALYST__
 			name = "MacCatalyst";
-			version = UIDevice.CurrentDevice.SystemVersion;
+			version = iOSSupportVersion;
 #elif __TVOS__ || __IOS__
 			name = UIDevice.CurrentDevice.SystemName;
 			version = UIDevice.CurrentDevice.SystemVersion;
@@ -92,6 +92,22 @@ namespace Xamarin.Tests
 			return platformInfo;
 		}
 
+#if __MACCATALYST__
+		static string? _iOSSupportVersion;
+		internal static string iOSSupportVersion {
+			get {
+				if (_iOSSupportVersion is null) {
+					// This is how Apple does it: https://github.com/llvm/llvm-project/blob/62ec4ac90738a5f2d209ed28c822223e58aaaeb7/lldb/source/Host/macosx/objcxx/HostInfoMacOSX.mm#L100-L105
+					using var dict = NSMutableDictionary.FromFile ("/System/Library/CoreServices/SystemVersion.plist");
+					using var str = (NSString) "iOSSupportVersion";
+					using var obj = dict.ObjectForKey (str);
+					_iOSSupportVersion = obj.ToString ();
+				}
+				return _iOSSupportVersion;
+			}
+		}
+#endif
+
 		public static readonly PlatformInfo Host = GetHostPlatformInfo ();
 
 #if NET