Map version for Tahoe compatibility. (#116536)

macOS Tahoe returns a compatibility version, 16, for macOS 26 unless it is built with Xcode 26's SDK.
As we did with Big Sur, this maps the compatibility version 16 to 26.

The intention is that we will be on the new SDK by the time macOS 27 rolls out. If not, then we will need to add another compatibility map, most likely.
It does not appear that iOS, tvOS, or Catalyst return compatibility numbers, so they are excluded from doing any mapping.

Author: vcsjones

src/libraries/Common/src/Interop/OSX/Interop.libobjc.cs

@@ -46,14 +46,17 @@ internal static Version GetOperatingSystemVersion()
                 }
             }
 
-            if (major == 10 && minor == 16)
+#if TARGET_OSX
+            if (major == 16)
             {
-                // We get "compat" version for 11.0 unless we build with updated SDK.
-                // Hopefully that will be before 11.x comes out
-                // For now, this maps 10.16 to 11.0.
-                major = 11;
-                minor = 0;
+                // MacOS Tahoe returns a compatibility version unless it is built with a new SDK. Map the compatibility
+                // version to the "correct" version. This assumes the minor versions will map unchanged.
+                // 16.0 => 26.0
+                // 16.1 => 26.1
+                // etc
+                major = 26;
             }
+#endif
 
             return new Version(major, minor, patch);
         }