Re-enable VSS in the default global policy

[mxk]

Background: #3543 (comment)

From what I can tell, everything is currently stable with the fixes in #3625 and #3629. Is that correct? If so, I would propose restoring when-available global policy default, and disabling VSS for individual tests that are sensitive to snapshot creation time (or maybe for all tests via testenv.NewCLITest?). I think Kopia should use VSS on Windows by default with it has the necessary permissions.

@jkowalski @julio-lopez @aaron-kasten @Shrekster What are your thoughts?

[basldfalksjdf]

Can we leave VSS off by default? Given how new the feature is, I don't think people should be forced to opt out but rather opt in.

[mxk]

It's opt-in for the current release. A proper implementation should be completely transparent to the operation of Kopia, and many people have already been using it for a long time via the PowerShell scripts that do effectively the same thing. I can't think of any cases where it would be preferable to create backups of a live file system rather than an immutable snapshot, so I think the default should eventually be switched to when-available.