Clean up minimum Windows version checks

src/MSBuild/Resources/Strings.resx

@@ -1092,7 +1092,7 @@ Copyright (C) Microsoft Corporation. All rights reserved.
     LOCALIZATION: The prefix "MSBUILD : error MSBxxxx:" should not be localized.</comment>
   </data>
   <data name="UnsupportedOS" UESanitized="true" Visibility="Public">
-    <value>MSBUILD : error MSB1015: MSBuild does not run on this version of the operating system. It is only supported on Windows 2000, Windows XP, and later versions.</value>
+    <value>MSBUILD : error MSB1015: MSBuild does not run on this version of the operating system. It is only supported on Windows 7 and later versions.</value>
     <comment>{StrBegin="MSBUILD : error MSB1015: "}LOCALIZATION: The error prefix "MSBUILD : error MSBxxxx:" should not be localized.</comment>
   </data>
   <data name="Using35Engine" UESanitized="true" Visibility="Public">

src/MSBuild/XMake.cs

@@ -1545,11 +1545,9 @@ private static bool BuildProjectWithOldOM(string projectFile, string[] targets,
         private static void VerifyThrowSupportedOS()
         {
 #if FEATURE_OSVERSION
-            if ((Environment.OSVersion.Platform == PlatformID.Win32S) ||        // Win32S
-                (Environment.OSVersion.Platform == PlatformID.Win32Windows) ||  // Windows 95, Windows 98, Windows ME
-                (Environment.OSVersion.Platform == PlatformID.WinCE) ||         // Windows CE
-                ((Environment.OSVersion.Platform == PlatformID.Win32NT) &&      // Windows NT 4.0 and earlier
-                (Environment.OSVersion.Version.Major <= 4)))
+            if (Environment.OSVersion.Platform != PlatformID.Win32NT ||
+                Environment.OSVersion.Version.Major < 6 ||
+                (Environment.OSVersion.Version.Major == 6 && Environment.OSVersion.Version.Minor < 1)) // Windows 7 is minimum
             {
                 // If we're running on any of the unsupported OS's, fail immediately.  This way,
                 // we don't run into some obscure error down the line, totally confusing the user.

src/Shared/EncodingUtilities.cs

@@ -219,16 +219,6 @@ internal static Encoding BatchFileEncoding(string contents, string encodingSpeci
 
             string useUtf8 = string.IsNullOrEmpty(encodingSpecification) ? EncodingUtilities.UseUtf8Detect : encodingSpecification;
 
-#if FEATURE_OSVERSION
-            // UTF8 is only supported in Windows 7 (6.1) or greater.
-            var windows7 = new Version(6, 1);
-
-            if (Environment.OSVersion.Version < windows7)
-            {
-                useUtf8 = EncodingUtilities.UseUtf8Never;
-            }
-#endif
-
             switch (useUtf8.ToUpperInvariant())
             {
                 case EncodingUtilities.UseUtf8Always: