Fix .NET 7 build warnings

Author: JimBobSquarePants

src/ImageSharp.Web/Caching/SHA256CacheHash.cs

@@ -56,14 +56,13 @@ public string Create(string value, uint length)
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     private static string HashValue(ReadOnlySpan<char> value, uint length, Span<byte> bufferSpan)
     {
-        using var hashAlgorithm = SHA256.Create();
         Encoding.ASCII.GetBytes(value, bufferSpan);
 
         // Hashed output maxes out at 32 bytes @ 256bit/8 so we're safe to use stackalloc
         Span<byte> hash = stackalloc byte[32];
-        hashAlgorithm.TryComputeHash(bufferSpan, hash, out int _);
+        SHA256.TryHashData(bufferSpan, hash, out int _);
 
         // Length maxes out at 64 since we throw if options is greater
-        return HexEncoder.Encode(hash.Slice(0, (int)(length / 2)));
+        return HexEncoder.Encode(hash[..(int)(length / 2)]);
     }
 }

src/ImageSharp.Web/Commands/Converters/ColorConverter.cs

@@ -79,7 +79,7 @@ public Color ConvertFrom(CommandParser parser, CultureInfo culture, string value
 
         // Named colors
         IDictionary<string, Color> table = ColorConstantsTable.Value;
-        return table.ContainsKey(value) ? table[value] : default;
+        return table.TryGetValue(value, out Color color) ? color : default;
     }
 
     private static IDictionary<string, Color> InitializeColorConstantsTable()