* Bump to .NET 6.0.100-alpha.1.21060.3.
* Fix dotnet command line arguments.
* dotnet build: the project file must be the first argument.
* dotnet build/publish: use the documented verbosity format.
* Update version number in tests.
* [tests/introspection] Adjust introspection to cope with different library names in the new .NET version.
* [tests/link sdk] Adjust the LinkSdkRegressionTest.SpecialFolder test according to the new version of .NET 6.
* [tests/link sdk] Preserve a required method in System.Private.CoreLib to work around a bug in .NET 6.
Ref: https://github.com/dotnet/runtime/issues/46908.
* Revert "[CI][VSTS] Add the donet 6 pkg as a dependency. (#10348)"
This reverts commit 6de4e717e7.
There's no need to provision .NET 6, it's done automatically.