Support for SVG icons in Redmine6

[takenory]

This pull request includes several updates to the user interface and compatibility improvements for the Redmine Issues Panel plugin. The most important changes include the addition of sprite icons to various UI elements and the update of the required Redmine version.

User Interface Enhancements:

• Added sprite icons to the "New Issue Category" and "New Version" links in the issue modal form (app/views/issues_panel/_modal_form.html.erb). [1] [2]
• Enhanced the query form by adding sprite icons to the filter and options legends and the apply, clear, save, and edit buttons (app/views/issues_panel/_query_form.html.erb). [1] [2]
• Updated the "New Issue" link in the issues panel index to use a sprite icon (app/views/issues_panel/index.html.erb).

Compatibility Improvements:

• Updated the required Redmine version to 6.0 in the plugin initialization file (init.rb).

Codebase Updates:

• Modified the render_issue_cards and render_issues_panel methods to include sprite icons for the "New Issue" button (lib/redmine/helpers/issues_panel.rb). [1] [2]

[hidakatsuya]

The changes in this pull request look good to me. I reviewed the code and verified the functionality.

[takenory]

Additionally, Replaced toggle icons with SVG when panels are grouped. b829ee2