Upgrade F# language version for 8.0

[kojo12228]

Also update another reference to C# language to 12 (the other one being amended in #3338).

Because the language version in compiler args is preview anyway, this doesn't matter too much (although it did surprise me that it was set to preview).

[jonsequitur]

Thanks, @kojo12228! This is a good catch.

The change you made doesn't affect the actual language version but it affects the language version that these two kernels advertise. There's probably a way (such as a test) to better keep these in sync in the future.

It incidentally caused one of the contract tests to fail.

interactive/src/Microsoft.DotNet.Interactive.ApiCompatibility.Tests/ApiCompatibilityTests.jupyter_api_is_not_changed.approved.txt

Line 241 in e89e1d0

.ctor(System.String version = 11.0)

This value should probably not be in the constructor for CSharpLanguageInfo and FSharpLanguageInfo since it's very much a moving target. But for now I'll hand-edit the approval file which should unblock this PR, and I'll open an issue for that afterwards.