text: Title: Remove support for singular section headers
  Body: Singular section headers were deprecated in RF 6.0 (#4431) and started to emit actual deprecation warnings in RF 7.0 (#4432). The earliest release we can remove their support for good is RF 8.0. If that's considered too early and we want to give users more time to update their data, removal can be postponed to RF 9.0.
  
  For reasons why we decided to remove the support for singular headers see #4431.