Support redmine-5.1

[nishidayuya]

Currently master (2852489) on redmine-5.1.3 occur LoadError as following:

yuya@yoshiyuki|16:08:00|0|12.7% bin/rails server
=> Booting Puma
=> Rails 6.1.7.8 application starting in development
=> Run `bin/rails server --help` for more startup options
Exiting
<internal:/home/yuya/.anyenv/envs/rbenv/versions/3.2.6/lib/ruby/3.2.0/rubygems/core_ext/kernel_require.rb>:38:in `require': cannot load such file -- google_analytics_hooks (LoadError)
        from <internal:/home/yuya/.anyenv/envs/rbenv/versions/3.2.6/lib/ruby/3.2.0/rubygems/core_ext/kernel_require.rb>:38:in `require'
        from /home/yuya/.anyenv/envs/rbenv/versions/3.2.6/lib/ruby/gems/3.2.0/gems/zeitwerk-2.7.1/lib/zeitwerk/core_ext/kernel.rb:34:in `require'
        from /home/yuya/.anyenv/envs/rbenv/versions/3.2.6/lib/ruby/gems/3.2.0/gems/activesupport-6.1.7.8/lib/active_support/dependencies/zeitwerk_integration.rb:51:in `require_dependency'
        from /home/yuya/src/github.com/redmine/redmine/plugins/google_analytics_plugin/init.rb:3:in `<top (required)>'
        from /home/yuya/src/github.com/redmine/redmine/lib/redmine/plugin_loader.rb:31:in `load'

This. pull-request fixes it.