move minimal dotnet version to 8

#!/bin/sh
 
case "$1" in
 
 'clean')
    dotnet bin/prebuild.dll /file prebuild.xml /clean
 
  ;;
 
 
  'autoclean')
 
    echo y|dotnet bin/prebuild.dll /file prebuild.xml /clean
 
  ;;
 
 
 
  *)
 
    cp bin/System.Drawing.Common.dll.linux bin/System.Drawing.Common.dll
    dotnet bin/prebuild.dll /target vs2022 /targetframework net6_0 /excludedir = "obj | bin" /file prebuild.xml
    echo "dotnet build -c Release OpenSim.sln" > compile.sh
    chmod +x compile.sh
 
  ;;
 
esac