Update TFM to net8.0

[wtgodbe]

Runtime has updated their TFM, so we need to do ours too. Ongoing work in #45475

Todo -

• Update branding & template precendence in spa-templates
• Update template precedence in aspnetcore
• Get a net8.0 SDK & do https://github.com/dotnet/aspnetcore/blob/main/docs/UpdatingMajorVersionAndTFM.md
• Update https://github.com/dotnet/aspnetcore/blob/main/docs/UpdatingMajorVersionAndTFM.md as discussed in Move templates to target net8.0 spa-templates#105 (comment). That is, we'll use 9{major}00 for 8.0 and 9.0, then {major}000 (aka {major} x 1000) forever after. If we did 8.1, the precedence should be 9810. If we did 10.1, 11000.
• Mark API from 7 as shipped
• Unskip Microbenchmark validation in Microsoft.AspNetCore.Server.HttpSys.Microbenchmarks

[radical]

What's the next step here?

[wtgodbe]

The TFM is already updated, I'm going to work on ingesting a net8.0 SDK today so we can get the last few things done. But for those who are ingesting new aspnetcore bits, everything should already be on net8.0.

[dougbu]

One bullet down but I also added a bullet covering our conversation in dotnet/spa-templates#105 (comment)

[wtgodbe]

Done