Switch to net7.0 tfm

[RussKie]

• dotnet/runtime and dotnet/aspnetcore have already switched over, e.g. https://github.com/dotnet/aspnetcore/blob/main/eng/Versions.props#L46. See dotnet/aspnetcore@5c4e769 for more details.
• remove all non dotnet7 NuGet feeds