Ensure we emit preview.1 versions as prerelease label. (#4909)

Author: joperezr

eng/Versions.props

@@ -3,7 +3,7 @@
     <MajorVersion>9</MajorVersion>
     <MinorVersion>0</MinorVersion>
     <PatchVersion>0</PatchVersion>
-    <PreReleaseVersionLabel>alpha</PreReleaseVersionLabel>
+    <PreReleaseVersionLabel>preview</PreReleaseVersionLabel>
     <PreReleaseVersionIteration>1</PreReleaseVersionIteration>
     <VersionPrefix>$(MajorVersion).$(MinorVersion).$(PatchVersion)</VersionPrefix>
     <ValidateBaseline>true</ValidateBaseline>