Upgrade .NET8 ZugferdTest

.github/upgrades/dotnet-upgrade-plan.md

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+# .NET 8.0 Upgrade Plan
+
+## Execution Steps
+
+Execute steps below sequentially one by one in the order they are listed.
+
+1. Validate that an .NET 8.0 SDK required for this upgrade is installed on the machine and if not, help to get it installed.
+2. Ensure that the SDK version specified in global.json files is compatible with the .NET 8.0 upgrade.
+3. Upgrade GUIs.Test.ZUGFeRDTest\ZUGFeRDTest.vbproj
+
+## Settings
+
+This section contains settings and data used by execution steps.
+
+### Excluded projects
+
+Table below contains projects that do belong to the dependency graph for selected projects and should not be included in the upgrade.
+
+| Project name | Description |
+|:-------------|:-----------:|
+| –            | Keine Projekte ausgeschlossen |
+
+### Project upgrade details
+This section contains details about each project upgrade and modifications that need to be done in the project.
+
+#### GUIs.Test.ZUGFeRDTest\ZUGFeRDTest.vbproj modifications
+
+Project properties changes:
+  - Keine Änderungen erforderlich. Das Projekt zielt bereits auf `net8.0-windows` und verwendet kompatible Pakete.