Disable Net5CompatFileStream by default

src/libraries/System.IO.FileSystem/tests/LegacyTests/LegacySwitchTests.cs

@@ -20,7 +20,7 @@ public static void LegacySwitchIsHonored()
                     .GetField("_strategy", BindingFlags.NonPublic | BindingFlags.Instance)
                     .GetValue(fileStream);
 
-                Assert.DoesNotContain(strategy.GetType().FullName, "Legacy");
+                Assert.Contains("Legacy", strategy.GetType().FullName);
             }
 
             File.Delete(filePath);

src/libraries/System.IO.FileSystem/tests/LegacyTests/runtimeconfig.template.json

@@ -1,5 +1,5 @@
 {
     "configProperties": {
-        "System.IO.UseNet5CompatFileStream": false
+        "System.IO.UseNet5CompatFileStream": true
     }
 }

src/libraries/System.IO/tests/LegacyTests/runtimeconfig.template.json

@@ -1,5 +1,5 @@
 {
     "configProperties": {
-        "System.IO.UseNet5CompatFileStream": false
+        "System.IO.UseNet5CompatFileStream": true
     }
 }

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/FileStreamHelpers.cs

@@ -19,7 +19,7 @@ private static bool GetLegacyFileStreamSetting()
 
             string? envVar = Environment.GetEnvironmentVariable("DOTNET_SYSTEM_IO_USENET5COMPATFILESTREAM");
             return envVar is null
-                ? true // legacy is currently enabled by default;
+                ? false // legacy is disabled by default;
                 : bool.IsTrueStringIgnoreCase(envVar) || envVar.Equals("1");
         }