Update release/3.1 branding to 3.1.15 (#4379)

Author: ryalanms

eng/Versions.props

@@ -9,7 +9,7 @@
       Version prefix used by a few projects like Project Templates
       Revision portion of Major.Minor.Revision usually updated every release
     -->
-    <TraditionalVersionPrefix>3.1.14</TraditionalVersionPrefix>
+    <TraditionalVersionPrefix>3.1.15</TraditionalVersionPrefix>
     <PreReleaseVersionLabel>servicing</PreReleaseVersionLabel>
     <DotNetUseShippingVersions>true</DotNetUseShippingVersions>
     <AssemblyVersion>4.0.0.0</AssemblyVersion>