Update dotnetcore.yml to install .NET 9

v3.4.0 - Drops building for .NET 6.0 which is beyond End of Support

v3.3.9 - Fixes issue #47 - Metafile parsing hack still may err but it…

… is only tried when lines have custom line caps now, what may not be that common

v3.3.8 - Cleanup more .net 7 leftovers

bump required sdk to 8.0.303

v3.3.7

   - Drop support for net7.0 who reached EOL
   - Upgrade System.Drawing.Common to 8.0.5 on needed target frameworks

v3.3.6 - .NET 8.0 and C# 12

v3.3.5 - Some changes on packaging readme file

v3.3.4 - Alternative fix for #72 (#72)

v3.3.3 - End of Life for net461: Bumping up to net462

v3.3.2 - Unit tests for SvgColor (which is now a bit more resilient a…

…lso)