Brand dotnet/runtime to NET 8 #72112

This work should be done as soon as possible when .NET 7 branched off and main starts targeting .NET 8.

• Branding (update package versions) so that packages don't collide with existing ones in the feed -> Bump main branding to 8.0 #73985
• Update the assembly version that will be hardcoded to 7.0, to 8.0. Update assembly version from hardcoded to MajorVersion and update NetCoreAppCurrent to net8.0 #78354
• Update projects to target net8.0 by updating the NetCoreAppCurrent property.
• Grepping for "net7.0" in the repo and replacing some of these to net8.0 (i.e. docs, yml, ...). Update assembly version from hardcoded to MajorVersion and update NetCoreAppCurrent to net8.0 #78354
• Decide if net7.0 assets must be kept, as packages would only include the minimum supported version (net6.0) and the current version (net8.0). Adding a third .NETCoreApp target would add significant cost to the build and make it slower.
• Add the dotnet8 feeds to the NuGet.config Add dotnet8 feeds in main nuget.config #74101
• At a later point when all dependencies are available in the .NET 8 feeds, remove the .NET 7 feeds. Remove dotnet7 feeds from NuGet.config when dependencies publish into dotnet8 feeds #63375

Mimic some of the change that I made last year when I updated NetCoreAppCurrent to net7.0.

cc @dotnet/runtime-infrastructure