Update branding to 18.3.0 (#15321)

Author: nohwnd

eng/Versions.props

@@ -14,7 +14,7 @@
       from appending +<commitId>, which breaks DTAAgent.
     -->
     <IncludeSourceRevisionInInformationalVersion>false</IncludeSourceRevisionInInformationalVersion>
-    <VersionPrefix>18.1.0</VersionPrefix>
+    <VersionPrefix>18.3.0</VersionPrefix>
     <PreReleaseVersionLabel>preview</PreReleaseVersionLabel>
   </PropertyGroup>
   <PropertyGroup Label="Arcade settings">