update branding from alpha to preview1 (#47282)

eng/Versions.props

@@ -6,7 +6,7 @@
     <MajorVersion>6</MajorVersion>
     <MinorVersion>0</MinorVersion>
     <PatchVersion>0</PatchVersion>
-    <PreReleaseVersionLabel>alpha</PreReleaseVersionLabel>
+    <PreReleaseVersionLabel>preview</PreReleaseVersionLabel>
     <PreReleaseVersionIteration>1</PreReleaseVersionIteration>
     <!-- Set assembly version to align with major and minor version,
          as for the patches and revisions should be manually updated per assembly if it is serviced. -->