This PR fixes #6106. Independent of whether we decide to enable these warnings in deal.II again, I think it would be a good idea to have them enabled for ASPECT. Otherwise our users do not notice if their plugins use deprecated features (this will likely already cause confusion because some of the deprecated features I removed in #6083 and #6084 may not have emitted any warnings with deal.II 9.5 or 9.6).
This PR fixes #6106. Independent of whether we decide to enable these warnings in deal.II again, I think it would be a good idea to have them enabled for ASPECT. Otherwise our users do not notice if their plugins use deprecated features (this will likely already cause confusion because some of the deprecated features I removed in #6083 and #6084 may not have emitted any warnings with deal.II 9.5 or 9.6).