Try to use delete_all instead of destroy_all

This may be a fix for the issue `Can't modify frozen hash` we get in the
pipeline during the unit tests.

spec/requests/media_spec.rb

@@ -15,7 +15,7 @@ def expect_all_results(response)
 RSpec.describe("Media", type: :request) do
   describe "#search_by" do
     before do
-      Medium.destroy_all
+      Medium.delete_all
 
       @medium1 = FactoryBot.create(:medium, :with_teachable, :with_editors, :released,
                                    sort: "Nuesse", description: "Erstes Medium")