[dotnet] Bump to preview 12. (#13466)

Make.config

@@ -80,7 +80,7 @@ endif
 
 # For release branches, modify the following variables to hardcode a version name
 # Set the NUGET_HARDCODED_PRERELEASE_IDENTIFIER variable to the prerelease identifer you want (say "preview.10")
-NUGET_HARDCODED_PRERELEASE_IDENTIFIER=preview.11
+NUGET_HARDCODED_PRERELEASE_IDENTIFIER=preview.12
 # Set the NUGET_HARDCODED_PRERELEASE_BRANCH variable to the exact name for the branch the above variable should apply to (so that any other branches won't pick it up by accident).
 # For the previous example, this would be "release/6.0.1xx-preview5"
 # When creating a release branch from main, this must be changed from "main" to the new release branch.