Evaluating 'ReadOnlySequence<T>.ToArray()' fails

## Environment data
C# Extension version: 1.21.9

## Steps to reproduce
* Create the following project.
* Set a breakpoint at the specified line
* Evaluate `sequence.ToArray()`

```
using System;
using System.Buffers;
using System.Text;

namespace DNC31Con
{
class Program
{
static void Main(string[] args)
{
byte[] bytes = { (Byte)'h', (Byte)'e', (Byte)'l', (Byte)'l', (Byte)'o', (Byte)'!' };
ReadOnlySequence sequence = new ReadOnlySequence(bytes);

Console.WriteLine("Hello World!"); // breakpoint here
}
}
}
```

## Expected behavior
Evaluation should succeed like it does in Visual Studio

## Actual behavior
`error CS8107: Feature 'ref extension methods' is not available in C# 7.0. Please use language version 7.2 or greater.`

## Notes
I believe the cause is that we need to upgrade the version of the C# EE that we ship in this extension to something more recent.