Support for Redmine6 (Rails7)

[cat-in-136]

1. Update Redmine/Ruby Versions in Workflows:

   • Updated the Redmine/Redmica/Ruby versions in the GitHub workflows to the latest stable releases.
   • Added new matrix environments for testing.

2. Ignore unloadable if Method Missing:

   • Modified the unloadable declaration in several models and controllers to conditionally check if the method is defined.

3. Use ApplicationRecord Instead of ActiveRecord::Base:

   • Refactored models to inherit from ApplicationRecord if available, ensuring compatibility with Rails7.

4. Workaround for LoadError:

   • Added a workaround in the GitHub workflow to handle the LoadError: cannot load such file -- blankslate issue for older versions.

5. Drop Support for Redmine 4.0 and 4.1:

   • Removed references and support for Redmine versions 4.0 and 4.1 from the workflows.

6. Update DateTime Handling:

   • Changed the way DateTime is formatted in fixtures to comply with Rails 7 standards.

7. Introduce Tabler Icons for Redmine 6.0:

   • Added new SVG icons and updated views to utilize the new icon set, matching redmine6 user interface.

8. Update README and init.rb:

   • Specified licenses in the README and updated the Redmine version requirement to 4.2.0. The plugin version has been bumped to 6.0.0.