Avoid span conversion in delegate extension receiver

[jjonescz]

An extension method Cᵢ.Mₑ is eligible if:

• Cᵢ is a non-generic, non-nested class
• The name of Mₑ is identifier
• Mₑ is accessible and applicable when applied to the arguments as a static method as shown above
• An implicit identity, reference ~~or boxing~~ , boxing, or span conversion exists from expr to the type of the first parameter of Mₑ. 👉 Span conversion is not considered when overload resolution is performed for a method group conversion.

Speclet update: https://github.com/dotnet/csharplang/pull/8221 Test plan: https://github.com/dotnet/roslyn/issues/73445