release/5.0 - Update SDK to final P8/RC1

[ViktorHofer]

We are currently using a non-final P8 SDK in release/5.0. We should update to a final P8/RC1 SDK at some point.

To consume the RC1 SDK, the following changes are necessary:

• Remove multi-targeting from runtime.depproj #41666
• Convert invalid C# uses of UnmanagedCallersOnly to IL. #42146
• Disable UnmanagedCallersOnlyTest on WASM. #42191
• Unpin C# compiler version #42174 (unpins Roslyn version)
• Fix for Almost 45000 warnings in coreclr Pri-1 test build #42184 once it is available

We should do the same change in master but as it impacts more developers, it should be done as part of a batched rollout (ideally next week's one).

cc @dotnet/runtime-infrastructure

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[ViktorHofer]

We believe this isn't necessary for shipping the product but we should do it anyway to minimize servicing diffs when taking a dependency on a serviced SDK.

[jkotas]

The release branch is building with Roslyn pinned to one of the preview versions. #42174 unpinned compiler in the master branch. Include this change in this port?

[ViktorHofer]

Our plan was to work on this post RTM before the first servicing release. Is there risk involved with keeping the existing pinned compiler? If yes, I could prepare a PR today to make this ready for RC2.

[jkotas]

This is a discussion for shiproom if there was not one.

I would more worried with the risk involved in updating the SDK and compiler. I would prefer to take it early when everyone is paying attention and problems are identified quickly, and not wait for the first servicing release. As I have said, it is a decision for the shiproom to make.

[jkotas]

I have added two more changes (#42146 + #42191) that need to be included in this.

[ViktorHofer]

@jkotas do you think that #42184 is required? Sounds to me that we introduce tons of warnings in our build without any noticeable customer impact.

[jkotas]

Sounds to me that we introduce tons of warnings in our build without any noticeable customer impact.

The CI impact and .NET team engineer impact is very noticeable.

[ViktorHofer]

Fixed by #42269