[release/7.0] Update baselines, et cetera

- update baselines to 7.0.0 release
  - start from <https://dev.azure.com/dnceng/_apis/resources/Containers/12084037/BlobArtifacts?itemPath=BlobArtifacts%2FMergedManifest.xml>
  - picks up all dropped and added packages in 7.0.0
- enable baseline validation
- update the .NET SDK to the GA version
- update site extension versions
- correct a typo in `$(DotNetPrivateAssetRootUrl)` value

Author: dougbu