Fix CI.

[peterhuene]

This commit:

• Updates target framework to net8.0.
• Fixes the externref API to match recent changes in upstream Wasmtime.
• Remove WASI "snapshot 0" from the tests (so old!).

[peterhuene]

"The process cannot access the file 'C:\Users\runneradmin\AppData\Local\Temp\tmpbr2aoc.tmp' because it is being used by another process."

Oh, Windows, never change.

[kpreisser]

"The process cannot access the file 'C:\Users\runneradmin\AppData\Local\Temp\tmpbr2aoc.tmp' because it is being used by another process."

FYI, on Windows it depends on which FileShare is specified when opening the file. When using File.ReadAllText(), it seems .NET specifies FileShare.Read, meaning opening the file fails when other code has opened the file for writing.
It should work if you manually specify FileShare.ReadWrite, which allows others to open the file for reading and writing, as long as they specify a Read file share (allowing us to open the file for reading).