release/6.0 should have PreRelease label 'preview' not 'alpha' (#1420)

Author: ryalanms

eng/Versions.props

@@ -4,7 +4,7 @@
     <MajorVersion>6</MajorVersion>
     <MinorVersion>0</MinorVersion>
     <PatchVersion>0</PatchVersion>
-    <PreReleaseVersionLabel>alpha</PreReleaseVersionLabel>
+    <PreReleaseVersionLabel>preview</PreReleaseVersionLabel>
     <PreReleaseVersionIteration>1</PreReleaseVersionIteration>
     <!-- Enable to remove prerelease label. -->
     <StabilizePackageVersion Condition="'$(StabilizePackageVersion)' == ''">false</StabilizePackageVersion>