Bump to RC 1

Make.config

@@ -133,7 +133,7 @@ endif
 ## change *first*, otherwise we'll produce builds with the same version from
 ## two different branches (which is very, very bad).
 ##
-NUGET_HARDCODED_PRERELEASE_IDENTIFIER=preview.7
+NUGET_HARDCODED_PRERELEASE_IDENTIFIER=rc.1
 NUGET_HARDCODED_PRERELEASE_BRANCH=net7.0
 
 # compute the alphanumeric version of branch names