Fix supported C# version in README

The C# Language class states it supports C# 6, the README previously
stated C# 8 is supported. Expression-bodied not read-only properties
(supported as of C# 7.0 [1]) like this don’t parse:

	public int foo { get => 4; }

which means the Language class is right and C# 8 isn’t fully supported.
Since the latest version is 11, and the first unsupported version (C# 7)
was released over six years ago, also reclassify C# support as “legacy”.

[1] https://github.com/dotnet/docs/blob/c802a0c56bccdc87908365c0788b3c5ecacea2b3/docs/csharp/programming-guide/statements-expressions-operators/expression-bodied-members.md

README.md

@@ -22,7 +22,7 @@ In the following, a list of all supported languages with their supported languag
 |--------------------------------------------------------|--------:|-------------------|:----------------------------------------------------------------:|:---------:|
 | [Java](https://www.java.com)                           |      17 | java              |                              mature                              |   JavaC   |
 | [C++](https://isocpp.org)                              |      11 | cpp               |                              legacy                              |  JavaCC   |
-| [C#](https://docs.microsoft.com/en-us/dotnet/csharp/)  |       8 | csharp            |                               beta                               |  ANTLR 4  |
+| [C#](https://docs.microsoft.com/en-us/dotnet/csharp/)  |       6 | csharp            |                              legacy                              |  ANTLR 4  |
 | [Go](https://go.dev)                                   |    1.17 | golang            |                               beta                               |  ANTLR 4  |
 | [Kotlin](https://kotlinlang.org)                       |     1.3 | kotlin            |                               beta                               |  ANTLR 4  |
 | [Python](https://www.python.org)                       |     3.6 | python3           |                              legacy                              |  ANTLR 4  |