PowerShell Core cannot start a job using Windows PowerShell

[daxian-dbw]

When -PSVersion is specified, it should be respected. And all supported PSVersions (2.0 to 5.1) should be supported when the corresopnding version of Windows PowerShell is available.

Note that, when -PSEdition 2.0 is specified, Start-Job checks if PSv2 is installed using the method RemotingCommandUtil.CheckIfPowerShellVersionIsInstalled which will blindly throw exception in PSCore (see code below). If we want to support -PSEdition 2.0, then the #if CORECLR section of the method body should be removed.

```c#
internal static void CheckIfPowerShellVersionIsInstalled(Version version)
{
// Check if PowerShell 2.0 is installed
if (version != null && version.Major == 2)
{

if CORECLR

            // PowerShell 2.0 is not available for CoreCLR
            throw new ArgumentException(
                PSRemotingErrorInvariants.FormatResourceString(
                    RemotingErrorIdStrings.PowerShellNotInstalled,
                    version, "PSVersion"));

else

            // Because of app-compat issues, in Win8, we will have PS 2.0 installed by default but not .NET 2.0
            // In such a case, it is not enough if we check just PowerShell registry keys. We also need to check if .NET 2.0 is installed.
            ...

Steps to reproduce
------------------

```powershell
PS:1> $job = Start-Job { $PSVersionTable.PSVersion } -PSVersion 5.1
PS:2> Receive-Job $job

Expected behavior

PS:2> Receive-Job $job

Major  Minor  Build  Revision
-----  -----  -----  --------
5      1      17134  81

Actual behavior

PS:2> Receive-Job $job

Major  Minor  Patch  PreReleas BuildLabel
                     eLabel
-----  -----  -----  --------- ----------
6      0      1

Environment data

> $PSVersionTable

Name                           Value
----                           -----
PSVersion                      6.0.1
PSEdition                      Core
GitCommitId                    v6.0.1
OS                             Microsoft Windows 10.0.17134
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

[BrucePay]

However, in PowerShell Core, we can only start job using pwsh. So the whole -PSVersion parameter doesn't really make sense.

I believe this is a bug (see #7052). Basically we've hard-coded what gets run when we start a job. There is no reason for this. We should be able to start a WindowsPowerShell job.

[daxian-dbw]

@BrucePay I have changed this issue to only track the Start-Job -PSVersion problem.
It turns out my comment about PSCore remoting to PSv2 server was not accurate. WMF 4.0 is required for setting up a PSCore endpoint, but it might not require WMF 4.0 for PSCore to connect to a PSv2 server. That means it's possible we still need to handle PSv2 server in remoting code.

[anmenaga]

This is fixed in 7.0.0-rc.1 for -PSVersion 5.1:

PS C:\> $PSVersionTable.PSVersion

Major  Minor  Patch  PreReleaseLabel BuildLabel
-----  -----  -----  --------------- ----------
7      0      0      rc.1

PS C:\> $job = Start-Job { $PSVersionTable.PSVersion } -PSVersion 5.1
PS C:\> Receive-Job $job

Major  Minor  Build  Revision
-----  -----  -----  --------
5      1      14393  3383

[daxian-dbw]

@anmenaga I think supporting -PSVersion 5.1 should be sufficient to close this bug, right?

[anmenaga]

@daxian-dbw I agree.