Update the .NET SDK in the CI builds to 6.0.300

[Numpsy]

Description

A minimal update to the .NET 6 SDK version, to see if it fixes the macOS/ARM CI failures discussed in #2778

[Numpsy]

Hmm, the macOS CI failed and the built output contains

Unhandled exception. System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
 ---> System.TypeInitializationException: The type initializer for '<StartupCode$Fake-DotNet-Cli-IntegrationTests>.$Fake.DotNet.Cli.IntegrationTests.TemplateTests' threw an exception.
 ---> System.DllNotFoundException: Unable to load shared library 'MonoPosixHelper' or one of its dependencies. In order to help diagnose loading problems, consider setting the DYLD_PRINT_LIBRARIES environment variable: dlopen(libMonoPosixHelper, 0x0001): tried: 'libMonoPosixHelper' (no such file), '/System/Volumes/Preboot/Cryptexes/OSlibMonoPosixHelper' (no such file), '/usr/lib/libMonoPosixHelper' (no such file, not in dyld cache), 'libMonoPosixHelper' (no such file), '/usr/local/lib/libMonoPosixHelper' (no such file), '/usr/lib/libMonoPosixHelper' (no such file, not in dyld cache)

So now I wonder if the old version of Mono.Posix it's using maybe only has Intel CPU support?

[Numpsy]

There are some integration tests in https://github.com/fsprojects/FAKE/blob/master/src/test/Fake.Core.IntegrationTests/Fake.DotNet.sdkAssemblyResolver.fs which are doing explicit things with the 6.0.10X SDK, and I start to wonder if that simply isn't going to work on ARM macs?

[Thorium]

This could help the CI failure: #2783

[xperiandri]

But it is already rebased

[Thorium]

Maybe the failure is this asking 6.0.100:
https://github.com/fsprojects/FAKE/blob/master/integrationtests/core-reference-assemblies-net60101-rollforward/before/global.json

...as the CI reports it here:
https://github.com/fsprojects/FAKE/actions/runs/10092902189/job/27907519123?pr=2780#step:8:9360

There are other too, e.g. here:
https://github.com/fsprojects/FAKE/blob/master/integrationtests/core-reference-assemblies-net60101/before/global.json

[Numpsy]

I tried updating some of those in https://github.com/Numpsy/FAKE/commits/6.0.131/ a while ago and hit some other failures.
At this point, I don't know if it's worth spending time updating the 6.0 SDKs or if it's better to put the effort into switching everything to .NET 8.0 before 6.0 goes out of support.

[xperiandri]

Probably 8 is better

[Numpsy]

The build at https://github.com/Numpsy/FAKE/actions/runs/10161143249 from the branch at https://github.com/Numpsy/FAKE/tree/bump_flail seems to have passed with the 6.0.30x SDK, though there are still a couple of changes n top of the SDK update in there