Use #public_send instead #method.call

These builder methods are using user provided input inside a public_send
but this is safe to do in this instance because before they are called
we check before calling them that they match an expected application
name.

app/services/clusters/applications/create_service.rb

@@ -10,8 +10,8 @@ module Clusters
       end
 
       def builder
-        cluster.method("application_#{application_name}").call ||
+        cluster.public_send(:"application_#{application_name}") || # rubocop:disable GitlabSecurity/PublicSend
-          cluster.method("build_application_#{application_name}").call
+          cluster.public_send(:"build_application_#{application_name}") # rubocop:disable GitlabSecurity/PublicSend
       end
     end
   end

app/services/clusters/applications/destroy_service.rb

@@ -16,7 +16,7 @@ module Clusters
       private
 
       def builder
-        cluster.method("application_#{application_name}").call
+        cluster.public_send(:"application_#{application_name}") # rubocop:disable GitlabSecurity/PublicSend
       end
     end
   end

app/services/clusters/applications/update_service.rb

@@ -10,7 +10,7 @@ module Clusters
       end
 
       def builder
-        cluster.method("application_#{application_name}").call
+        cluster.public_send(:"application_#{application_name}") # rubocop:disable GitlabSecurity/PublicSend
       end
     end
   end