Update to .NET 9 rc1

[vzarytovskii]

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[vzarytovskii]

FYI @edgarfgp @auduchinok @DedSec256 @Martin521 @brianrourkeboll

Plain builds are unfortunately currently broken if building without /p:FSHARPCORE_USE_PACKAGE=False, since it is now using a shipped FSharp.Build which has task which generates some of the files (like FSBuild.fs or fsi settings), and uses some of the nullability features (objnull, nonNull, etc).

Please use build scripts instead.

We want to upgrade to rc1, since it has a bunch of quite important fixes, which we want to get in.

Update: I have fixed most of issues, but there's currently no way around of using shipped FSharp.Build (and inherently FSharp.Core project). This is a current workaround until we ship .NET 9 rc2 or until I will figure out the way around it.

Update 2: #17700 should fix it for newer compiler + older fscore when we update to rc2.

[vzarytovskii]

I have fixed most of issues, but there's currently no way around of using shipped FSharp.Build (and inherently FSharp.Core project).

[Martin521]

Locally, everything still works fine for me.
I have seen these nullness errors before, but never was able to repro them reliably.

[vzarytovskii]

Locally, everything still works fine for me. I have seen these nullness errors before, but never was able to repro them reliably.

It is not yet merged. This PR was breaking it by default, i "fixed" it, but it will use fsharp.core project by default as opposed to library.

[vzarytovskii]

Either way, I think rc1 update will have to wait, we're hitting
dotnet/core#9489

[vzarytovskii]

That'd be a "workaround" to unblock plain builds, but it will by default use the project.

[vzarytovskii]

Turning it off currently for the plain builds, so we don't get mismatches with defines we have.