build: pin net6 package dependencies to 8.0.0

[iamherrylok]

Summary

This PR adjusts package versions specifically for .NET 6 targets while keeping current versions for newer frameworks.

Changes

• src/Clowd.Clipboard/Clowd.Clipboard.csproj

  • Use System.Text.Encoding.CodePages 8.0.0 when TargetFramework starts with net6.0 (covers both net6.0 and net6.0-windows).
  • Keep System.Text.Encoding.CodePages 9.0.0 for non-.NET 6 target frameworks.

• src/Clowd.Clipboard.Gdi/Clowd.Clipboard.Gdi.csproj

  • Use System.Drawing.Common 8.0.0 when TargetFramework starts with net6.0 (covers both net6.0 and net6.0-windows).
  • Keep System.Drawing.Common 9.0.0 for non-.NET 6 target frameworks (excluding net461 as before).

Why

.NET 6 targets should consume dependency versions aligned with the .NET 8 package line (8.0.0) rather than 9.0.0.

This keeps behavior for .NET 8+ targets unchanged while improving compatibility and clarity for .NET 6 target selection.

Notes

• Condition logic uses StartsWith("net6.0") to cover both net6.0 and net6.0-windows in a single expression.
• No runtime behavior changes; this is a package-version targeting update only.

[caesay]

Hey, thanks for the contribution. Not sure I entirely understand why this is necessary? Your entire PR description (including the Why) talks about what was done rather than why it is needed.

[iamherrylok]

hello caesay, sorry that my earlier description wasn’t very clear.

When using Clowd.Clipboard.Gdi in a .NET 6 project, I consistently get warnings because System.Drawing.Common 9.0.0 and System.Text.Encoding.CodePages 9.0.0 are no longer supported on net6.0. This PR pins those dependencies to version 8.0.0 when targeting .NET 6 in order to eliminate those warnings and keep the package compatible with that target framework.