Fix Redmine 5/Rails 6.1 compatibility

[tgoeg]

First off: Thanks for taking over maintainership for this plugin. Very much appreciated. Warning: I am just a sysadmin with no proper coding experience in Ruby. These are just fixes that seem syntactically correct to me and work for us. They might be wrong. Any review and improvement is very welcome!

This fixes compatibility for us with Redmine 5.0.5, both for the webGUI and the rake task.

Untested with Redmine 4.2 and older Rails versions. I don't know if the zeitwerk-related changes are backward compatible. If not, I won't be able to fix this with conditionals, as my Ruby is too bad for this.