Change Copy task LogDiagnostics Warning so it is suppressible

[rokonec]

Fixes #9210

Context

Copy task implement LogDiagnostics as warnings. This is causing issues for /WarnAsError use cases.

Changes Made

Change Copy task LogDiagnostics from Warning to low importance Message.

/// <summary>
/// If MSBUILDALWAYSRETRY is set, also log useful diagnostic information -- as 
/// a warning, so it's easily visible. 
/// </summary>
private void LogDiagnostic(string message, params object[] messageArgs)
{
    if (s_alwaysRetryCopy)
    {
        Log.LogMessage(MessageImportance.Low, message, messageArgs);
    }
}

The reasoning for Warning level "so it's easily visible" is, IMO, not strong enough.

Testing

Unit tests.
Locally.

Notes

@rainersigwald Since we see increasing number of people reporting issues with Copy conflicts, I believe it is candidate for 17.8

[JanKrivanek]

Please remove/alter the existing comment (it's confusing now) - than I'm OK with change. As soon as the MessageImportance.High was considered and there are reasons not to use it.

[rokonec]

Not closed actually, but hard reset + force push have confused github

[JanKrivanek]

Looks good!

We just need to pick unique error codes