Update Pong with C# to Godot 4.2.1

[behroozbc]

I saw the mono pong demo had been version 4 dev. which cannot build in version 4 because change on dotnet version (4.7 to 6).
I update and test it.
I did not make any issue.

[Calinou]

Tested locally on 4.2.1, it works as expected. I pushed some additional changes to the PR, such as using integer scaling, disabling filtering on sprites, using physical key inputs and using the Compatibility rendering method.

Thanks!