use new System.Text.Ascii APIs, remove internal helpers

src/Http/Headers/src/ContentDispositionHeaderValue.cs

@@ -631,7 +631,7 @@ private static string Encode5987(StringSegment input)
         int totalBytesConsumed = 0;
         while (totalBytesConsumed < inputBytes.Length)
         {
-            if (inputBytes[totalBytesConsumed] <= 0x7F)
+            if (Ascii.IsValid(inputBytes[totalBytesConsumed]))
             {
                 // This is an ASCII char. Let's handle it ourselves.
 

src/Http/Routing/src/Matching/JumpTableBuilder.cs

@@ -3,6 +3,7 @@
 
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
+using System.Text;
 
 namespace Microsoft.AspNetCore.Routing.Matching;
 
@@ -42,7 +43,7 @@ public static JumpTable Build(int defaultDestination, int exitDestination, (stri
 
         // The IL Emit jump table is not faster for a single entry - but we have an optimized version when all text
         // is ASCII
-        if (pathEntries.Length == 1 && Ascii.IsAscii(pathEntries[0].text))
+        if (pathEntries.Length == 1 && Ascii.IsValid(pathEntries[0].text))
         {
             var entry = pathEntries[0];
             return new SingleEntryAsciiJumpTable(defaultDestination, exitDestination, entry.text, entry.destination);

src/Http/Routing/src/Matching/SingleEntryAsciiJumpTable.cs

@@ -1,6 +1,9 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics;
+using System.Text;
+
 namespace Microsoft.AspNetCore.Routing.Matching;
 
 // Optimized implementation for cases where we know that we're
@@ -41,7 +44,9 @@ public override int GetDestination(string path, PathSegment segment)
         var a = path.AsSpan(segment.Start, length);
         var b = text.AsSpan();
 
-        return Ascii.AsciiIgnoreCaseEquals(a, b, length) ? _destination : _defaultDestination;
+        Debug.Assert(a.Length == b.Length && b.Length == length);
+
+        return Ascii.EqualsIgnoreCase(a, b) ? _destination : _defaultDestination;
     }
 
     public override string DebuggerToString()

src/Servers/Kestrel/Core/src/Internal/Infrastructure/HttpUtilities.cs

@@ -54,7 +54,10 @@ private static ulong GetAsciiStringAsLong(string str)
     {
         Debug.Assert(str.Length == 8, "String must be exactly 8 (ASCII) characters long.");
 
-        var bytes = Encoding.ASCII.GetBytes(str);
+        Span<byte> bytes = stackalloc byte[8];
+        OperationStatus operationStatus = Ascii.FromUtf16(str, bytes, out _);
+
+        Debug.Assert(operationStatus == OperationStatus.Done);
 
         return BinaryPrimitives.ReadUInt64LittleEndian(bytes);
     }
@@ -63,7 +66,11 @@ private static uint GetAsciiStringAsInt(string str)
     {
         Debug.Assert(str.Length == 4, "String must be exactly 4 (ASCII) characters long.");
 
-        var bytes = Encoding.ASCII.GetBytes(str);
+        Span<byte> bytes = stackalloc byte[4];
+        OperationStatus operationStatus = Ascii.FromUtf16(str, bytes, out _);
+
+        Debug.Assert(operationStatus == OperationStatus.Done);
+
         return BinaryPrimitives.ReadUInt32LittleEndian(bytes);
     }