PR Summary

Changes are:

1. The change log for v7.1.1 release.
2. Remove unneeded feeds from test/hosting/NuGet.Config, so that the SDK test can work in release pipeline.
3. Remove old .NET SDKs if any exists. The macOS builder has a lot old .NET SDKs installed on it and it seems the latest .NET 5 SDK has a compatible issue with one of them, which made dotnet restore fail. This happened on macOS only.

PR Checklist

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