Bug description

Source generated resolver requires C# 9. .netstandard 2.0 project needs to add <LangVersion>9.0</LangVersion> to avoid compilation error. There are two parts of code causing this issue:

1. The following code causes "Feature 'target-type object creation' is not available in C# 7.3. Please use language version 9.0 or greater."

private static readonly global::System.Collections.Generic.Dictionary<global::System.Type, int> closedTypeLookup = new(24)
{
    // ...
}

3. The following code causes "Feature 'recursive patterns' is not available in C# 7.3. Please use language version 8.0 or greater."

return closedKey switch
{
  0 => new MsgPack::Formatters.ListFormatter<int>(),
  // ...
}

Repro steps

Use MessagePack 3.x in a .netstandard 2.0 project

Expected behavior

.netstandard 2.0 doesn't need to change language version.

Actual behavior

.netstandard 2.0 project needs to add <LangVersion>9.0</LangVersion> to avoid compilation error.

• Version used: 3.0.134-beta
• Runtime: .NET Core