Upgrade to net6.0 to prevent dependency issues after upgrading MSBuild.Locator

[carlossanlop]

Related to #40
cc @safern @ericstj

[carlossanlop]

The CI fails with: The current .NET SDK does not support targeting .NET 6.0.. Do you know if .NET 6.0 can be used officially in GitHub CI?
Let's find out with a new commit.

[ericstj]

Did you make a change to your global.json to pre-requisite this version? Does your repo use Arcade? That should acquire a CLI for building based on global.json contents.

[safern]

Does your repo use Arcade? That should acquire a CLI for building based on global.json contents.

No it doesn't use arcade, but we should add a global.json file with a rollForward policy.

[carlossanlop]

The CI error is weird. I assume it's saying it didn't recognize 6.0.x as a .NET version?

[safern]

I can't push to this branch. The CI error is because when the latest version available in the releases-json index is a preview version, their semver check is failing. So you either need to specify an exact version i.e the full preview version of the SDK or since we need to add a global.json file anyway, don't include a version and that will fallback to the global.json specified version.

Here is a PR to fix that issue in their action by taking an include prerelease parameter: actions/setup-dotnet#110

I put up a PR because I can't push to your branch which adds global.json and the build is green. #46