Support .NET

[RussKie]

Git Extensions@master has been migrated to .NET 5.0. Once .NET 6.0 is released the app will be migrated to it.
The plugin manager is currently can't be used with the dev version of the app as it is targeting .NET Framework.

[maraf]

I have prepared migration to .NET 5. Most of the things were smooth. What doesn't work yet

• Single file exe using Costura.Fody doesn't start at all Replaced with .NET's single file publish.
• Portable GE (3.6.0.12890) doesn't load the plugin from UserPugins folder (Fix loading user plugins. gitextensions#9661).
• GitExtensions.Extensibility should target .NET 5.0 (
  Add support for targeting .NET 5 plugins. gitextensions.extensibility#21).
• Changing nuget source order doesn't work (after nuget package update).
• Update Prepare-Release.ps1 and Zip-GitExtensionsPlugin.ps1 scripts.

[RussKie]

I think it would be prudent to bump the major version to 2.0.0 for the release