Add support for Redmine 5.x

[perkooo]

It seems that there is some incompatibility with Redmine 5.x
We currently can't install the plugin on a Redmine 5.x version:

/usr/share/rubygems-integration/all/gems/bundler-1.17.3/lib/bundler/rubygems_integration.rb:200: warning: constant Gem::ConfigMap is deprecated
/usr/share/rubygems-integration/all/gems/bundler-1.17.3/lib/bundler/rubygems_integration.rb:200: warning: constant Gem::ConfigMap is deprecated
rake aborted!
LoadError: cannot load such file -- redmine_mattermost/listener
/var/www/redmine-5.0.2/vendor/bundle/ruby/2.5.0/gems/zeitwerk-2.6.0/lib/zeitwerk/kernel.rb:35:in `require'
/var/www/redmine-5.0.2/vendor/bundle/ruby/2.5.0/gems/zeitwerk-2.6.0/lib/zeitwerk/kernel.rb:35:in `require'
/var/www/redmine-5.0.2/vendor/bundle/ruby/2.5.0/gems/activesupport-6.1.6/lib/active_support/dependencies/zeitwerk_integration.rb:51:in `require_dependency'