Migrate the template to .NET

[RussKie]

The template is still targeting .NET Framework, which will no longer work for Git Extensions v4+ (it's targeting .NET 6.0).

gitextensions.plugintemplate/src/GitExtensions.PluginTemplate/GitExtensions.PluginTemplate.csproj

Line 4 in 965d08e

<TargetFramework>net461</TargetFramework>

Update the template to target the .NET 6.0 (or whatever TFM is currently targeted by Git Extensions). https://github.com/gitextensions/GitExtensions.GerritPlugin may act as a good reference source.