List of known changes we should do in Yarp once adding a 6.0 target (and/or more convenient after dropping 3.1):

• Use non-validated header enumeration API Proposal: Non-validated HttpHeaders enumeration dotnet/runtime#35126 (comment)
• Call Cancellable/Sync overloads of HttpContent.ReadAsStream* Pass CancellationToken to HttpContent.ReadAsStreamAsync for .Net 5.0 #770
• Doc Kestrel response header encoding dotnet/aspnetcore#33776 here: https://github.com/microsoft/reverse-proxy/blob/16ab855a5be980a0cf5fd21e9a7028190b6e7dda/docs/docfx/articles/http-client-config.md#httpclient
• CancellationTokenSource pooling Pool CancellationTokenSources in 6.0+ #902
• Remove ActivityPropagationHandler in favour of the DistributedContextPropagator that can be customized on a per-handler basis Allow specifying a context propagator on SocketsHttpHandler dotnet/runtime#55556
• Use Random.Shared in RandomFactory
• Update NameResolutionMetrics to include current-dns-lookups Add current-dns-lookups counter to System.Net.NameResolution dotnet/runtime#49146
• Implement the cancellable overload of HttpContent.SerializeToStreamAsync (5.0) (Always wait for the request content to complete #1271)
• Use WaitAsync task timeout extensions instead of TimeoutAfter

Feel free to add anything I may be missing