Multi-target net48 and net7.0, streamline Docker image

[HebaruSan]

• The Docker image now starts from ubuntu:latest instead of the rapidly aging mono:latest and installs only the parts of Mono it needs, which dramatically shrinks it:

  REPOSITORY                         TAG       IMAGE ID       CREATED         SIZE
  kspckan/kspmmcfgvalidator          latest    123869981b6e   16 months ago   788MB
  
  REPOSITORY                         TAG       IMAGE ID       CREATED         SIZE
  kspmmcfgvalidator_testing          latest    0cb2723a6dd9   5 seconds ago   196MB
  

• The .NET Framework target version is updated to 4.8
• A .NET 7.0 target is added (but not used for anything yet)
• ILRepack's noisy output is hidden during the build
• The Resources.Designer.cs file is now auto-generated at build time