Fix .NET SDK version needed by berp in CONTRIBUTING.md

Author: gasparnagy

CONTRIBUTING.md

@@ -33,7 +33,7 @@ docker run --rm --interactive --tty --volume ".:/app" berp-env
 
 *Or* install on your system
 
-* .NET 5.0 (to run `berp` to generate parsers)
+* .NET 8.0 (to run `berp` to generate parsers)
 * `berp` (install with `dotnet tool update Berp --version 1.4.0 --tool-path /usr/bin` )
 * `make`
 * `diff`