[net9.0] We've branched preview 6, so we're preview 7 now.

Make.config

@@ -134,7 +134,7 @@ endif
 ##
 ## Note that the prerelease identifier should be as short as possible, because otherwise
 ## the resulting package name can become too long for MSIs.
-NUGET_HARDCODED_PRERELEASE_IDENTIFIER=net9-p6
+NUGET_HARDCODED_PRERELEASE_IDENTIFIER=net9-p7
 NUGET_HARDCODED_PRERELEASE_BRANCH=net9.0
 
 # compute the alphanumeric version of branch names